docs: mention "-k" for both forms of "git mv"

The "git mv" synopsis shows two forms: renaming a file, and
moving files into a directory. They can both make use of the
"-k" flag to ignore errors, so mention it in both places.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>