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>

Documentation/git-mv.txt

@@ -15,7 +15,7 @@ DESCRIPTION
 -----------
 This script is used to move or rename a file, directory or symlink.
 
- git mv [-f] [-n] <source> <destination>
+ git mv [-f] [-n] [-k] <source> <destination>
  git mv [-f] [-n] [-k] <source> ... <destination directory>
 
 In the first form, it renames <source>, which must exist and be either