Fix typo in period flag in cli.md (#2491)

Fix typo in period flag description in cli.md

Author: lihongong

docs/ug/cli.md

@@ -201,7 +201,7 @@ credit is given.
 ### `--period`, `-p`
 
 **`--period PERIOD`**: Specifies the period of analysis window.
-* Parameter: `PERIOD` The period of analysis window, in the format `nd` (for n days) or `nw` (for n weeks). It is used to calculate end date if only start date is specified, or calculate end date if only start date is specified.
+* Parameter: `PERIOD` The period of analysis window, in the format `nd` (for n days) or `nw` (for n weeks). It is used to calculate end date if only start date is specified, or calculate start date if only end date is specified.
 * Alias: `-p`
 * Example: `--period 30d` or `--period 4w`