bug#44959: date error message should say -I

[積丹尼 Dan Jacobson]

$ date -I=seconds
date: invalid argument ‘=seconds’ for ‘--iso-8601’

Hey, that is a valid argument for --iso-8601. (But not for -I, so say
that instead.)

Here is a real invalid argument:

$ date --iso-8601=secondsz
date: invalid argument ‘secondsz’ for ‘--iso-8601’

date (GNU coreutils) 8.32