Fix example for post create command

[ernilambar]

Current:

# Create post and schedule for future
$ wp post create --post_type=page --post_title='A future post' --post_status=future --post_date='2020-12-01 07:00:00'

Example command does not match with its description.

Changed to:

# Create post and schedule for future
$ wp post create --post_type=post --post_title='A future post' --post_status=future --post_date='2030-12-01 07:00:00'