Fix time format for TAI Timestamps (#44)

In the documentation, `X` / `x` was used for both, 'UTC' timestamp and 'TAI' timestamp. However, according to the code, `XT` / `xt` must be used to format a time as TAI timestamp:

https://github.com/kshetline/tubular_time/blob/8341db46c34c7d067b8accf560d19532fd2f3268/src/format-parse.ts#L497

Author: sedrubal

README.md

@@ -266,8 +266,8 @@ For example:
 | Unix millisecond timestamp, UTC   |                             x | 1360013296123                                                                                                                                                                                                                                                                                                        |
 | Unix timestamp, epoch             |                            XX | 1360013296                                                                                                                                                                                                                                                                                                           |
 | Unix millisecond timestamp, epoch |                            xx | 1360013296123                                                                                                                                                                                                                                                                                                        |
-| Unix timestamp, TAI               |                             X | 1360013331                                                                                                                                                                                                                                                                                                           |
-| Unix millisecond timestamp, TAI   |                             x | 1360013331123                                                                                                                                                                                                                                                                                                        |
+| Unix timestamp, TAI               |                            XT | 1360013331                                                                                                                                                                                                                                                                                                           |
+| Unix millisecond timestamp, TAI   |                            xt | 1360013331123                                                                                                                                                                                                                                                                                                        |
 | Daylight Saving Time indicator    |                             V | § # ^ ~ ❄<br><br>Symbol indicating DST is in effect.<br>This is typically §, meaning the clock has been turned forward one hour.<br># means two hours forward, ^ means half an hour, ~ is any other forward amount.<br>❄ is negative DST, i.e. “Winter Time”.<br>Renders one blank space when DST is not in effect.  |
 |                                   |                             v | Same as above, but no blank space when DST is not in effect.                                                                                                                                                                                                                                                         |
 | Occurrence indicator              |                             R | 1:00 , 1:01 ... 1:58 , 1:59 , 1:00₂, 1:01₂ ... 1:58₂, 1:59₂, 2:00 , 2:01<br><br>A subscript 2 (₂) that denotes the second occurrence of the same clock time during a day when clocks are turned back for Daylight Saving Time.                                                                                       |