stash doc: mention short form -k in save description

Document --keep-index's short form -k in both main synopsis and
the save synopsis in the Options section.
Signed-off-by: NJohn Marshall <jm18@sanger.ac.uk>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-stash.txt

@@ -44,7 +44,7 @@ is also possible).
 OPTIONS
 -------
 
-save [-p|--patch] [--[no-]keep-index] [-u|--include-untracked] [-a|--all] [-q|--quiet] [<message>]::
+save [-p|--patch] [-k|--[no-]keep-index] [-u|--include-untracked] [-a|--all] [-q|--quiet] [<message>]::
 
 	Save your local modifications to a new 'stash', and run `git reset
 	--hard` to revert them.  The <message> part is optional and gives