Documentation/fmt-merge-msg: fix markup in example

Use at least 4 delimiting dashes that are required for
ListingBlock to get this block rendered as verbatim text.

Signed-off-by: Stefan Christ <contact@stefanchrist.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fmt-merge-msg.txt

@@ -60,10 +60,10 @@ merge.summary::
 EXAMPLE
 -------
 
---
+---------
 $ git fetch origin master
 $ git fmt-merge-msg --log <$GIT_DIR/FETCH_HEAD
---
+---------
 
 Print a log message describing a merge of the "master" branch from
 the "origin" remote.