Documentation/git-merge.txt: fix formatting of example block

You need at least four dashes in a line to have it recognized as listing
block delimiter by asciidoc.

Signed-off-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge.txt

@@ -187,11 +187,11 @@ In such a case, you can "unwrap" the tag yourself before feeding it
 to `git merge`, or pass `--ff-only` when you do not have any work on
 your own. e.g.
 
----
+----
 git fetch origin
 git merge v1.2.3^0
 git merge --ff-only v1.2.3
----
+----
 
 
 HOW CONFLICTS ARE PRESENTED