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

[Jim Meyering]

address@hidden wrote:
> $ 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

Thanks for the report.
However, that doesn't lend itself well to a clean fix, since +FORMAT
is not a literal string option argument like the others, and currently
that diagnostic is generated automatically based on the list of literal strings.

I'm inclined to leave it as-is.

Any other opinions?