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

[jidanni]

>>>>> "JM" == Jim Meyering <j...@meyering.net> writes:
JM> Hence, including that prefix shows what I've done.  Without it,
JM> the reader would wonder if I'd simply used whatever ls is in my path,
JM> which could easily represent a mistake.
Well OK then.