parseopt: fix documentation for --keep-dashdash

Signed-off-by: NUwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-rev-parse.txt

@@ -26,7 +26,7 @@ OPTIONS
 --parseopt::
 	Use 'git-rev-parse' in option parsing mode (see PARSEOPT section below).
 
---keep-dash-dash::
+--keep-dashdash::
 	Only meaningful in `--parseopt` mode. Tells the option parser to echo
 	out the first `--` met instead of skipping it.