Documentation: fix typo in rev-parse --short option description.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-rev-parse.txt

@@ -80,7 +80,7 @@ OPTIONS
 --git-dir::
 	Show `$GIT_DIR` if defined else show the path to the .git directory.
 
---short, short=number::
+--short, --short=number::
 	Instead of outputting the full SHA1 values of object names try to
 	abbriviate them to a shorter unique name. When no length is specified
 	7 is used. The minimum length is 4.