builtin/repack.c: fix incorrect reference to '-C'

builtin/repack.c: fix incorrect reference to '-C'

When cruft packs were originally being developed, `-C` was designated as
the short-form for `--cruft` (as in `git repack -C`).

This was dropped due to confusion with Git's top-level `-C` option
before submitting to the list. But the reference to it in
`--cruft-expiration`'s help text was never updated. Fix that dangling
reference in this patch.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>