Fix typo in pg_partman.md (#766)

Author: yoshihikoueno

doc/pg_partman.md

@@ -212,7 +212,7 @@ RETURNS boolean
     + *\<interval\>*      - Any valid value for the interval data type. Do not type cast the parameter value, just leave as text.
     + *\<integer\>*       - For ID based partitions, the integer value range of the ID that should be set per partition. Enter this as an integer in text format ('100' not 100). If the interval is >=2, then the `p_type` must be `range`. If the interval equals 1, then the `p_type` must be `list`. Also note that while numeric values are supported for id-based partitioning, the interval must still be a whole number integer.
  * `p_type` - the type of partitioning to be done. Currently only **range** and **list** are supported. See `p_interval` parameter for special conditions concerning type.
- * `p_epoch` - tells `pg_partman` that the control column is an integer type, but actually represents and epoch time value. Valid values for this option are: 'seconds', 'milliseconds', 'microseconds', 'nanoseconds', and 'none'. The default is 'none'. All table names will be time-based. In addition to a normal index on the control column, be sure you create a functional, time-based index on the control column (to_timestamp(controlcolumn)) as well so this works efficiently.
+ * `p_epoch` - tells `pg_partman` that the control column is an integer type, but actually represents an epoch time value. Valid values for this option are: 'seconds', 'milliseconds', 'microseconds', 'nanoseconds', and 'none'. The default is 'none'. All table names will be time-based. In addition to a normal index on the control column, be sure you create a functional, time-based index on the control column (to_timestamp(controlcolumn)) as well so this works efficiently.
  * `p_premake` - is how many additional partitions to always stay ahead of the current partition. Default value is 4. This will keep at minimum 5 partitions made, including the current one. For example, if today was Sept 6th, and `premake` was set to 4 for a daily partition, then partitions would be made for the 6th as well as the 7th, 8th, 9th and 10th. Note some intervals may occasionally cause an extra partition to be premade or one to be missed due to leap years, differing month lengths, etc. This usually won't hurt anything and should self-correct (see **About** section concerning timezones and non-UTC). If partitioning ever falls behind the `premake` value, normal running of `run_maintenance()` and data insertion should automatically catch things up.
  * `p_start_partition` - allows the first partition of a set to be specified instead of it being automatically determined. Must be a valid timestamp (for time-based) or positive integer (for id-based) value. Be aware, though, the actual parameter data type is text. For time-based partitioning, all partitions starting with the given timestamp up to CURRENT_TIMESTAMP (plus `premake`) will be created. For id-based partitioning, only the partition starting at the given value (plus `premake`) will be made. Note that for subpartitioning, this only applies during initial setup and not during ongoing maintenance.
  * `p_default_table` - boolean flag to determine whether a default table is created. Defaults to true.