bug#10253: mention +FORMAT in ls time style reminder help blurb

[jidanni]

$ ls -t1 --time-style=%c -og
ls: invalid argument `%c' for `time style'
Valid arguments are:
  - `full-iso'
  - `long-iso'
  - `iso'
  - `locale'     <-------------you forgot to also mention "+FORMAT"
Try `ls --help' for more information.
$ ls -t1 --time-style=+%c -og