merge: tweak --rerere-autoupdate documentation

Spell out --no-rerere-autoupdate explictly to make searching
easier. This matches the other --no options in the man page.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge.txt

@@ -83,7 +83,8 @@ invocations. The automated message can include the branch description.
 If `--log` is specified, a shortlog of the commits being merged
 will be appended to the specified message.
 
---[no-]rerere-autoupdate::
+--rerere-autoupdate::
+--no-rerere-autoupdate::
 	Allow the rerere mechanism to update the index with the
 	result of auto-conflict resolution if possible.