Fix a formatting error in git-merge.txt

Inside an element of an enumerated list, the second and subsequent
paragraphs need to lose their indent and have to be strung together with a
line with a single '+' on it instead.  Otherwise the lines below are shown
in typewriter face, which just looks wrong.

Signed-off-by: Nathan W. Panike <nathan.panike@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge.txt

@@ -59,13 +59,13 @@ include::merge-options.txt[]
 -m <msg>::
 	Set the commit message to be used for the merge commit (in
 	case one is created).
-
-	If `--log` is specified, a shortlog of the commits being merged
-	will be appended to the specified message.
-
-	The 'git fmt-merge-msg' command can be
-	used to give a good default for automated 'git merge'
-	invocations.
++
+If `--log` is specified, a shortlog of the commits being merged
+will be appended to the specified message.
++
+The 'git fmt-merge-msg' command can be
+used to give a good default for automated 'git merge'
+invocations.
 
 --rerere-autoupdate::
 --no-rerere-autoupdate::