Fixed link in user-manual

link to git-mergetool was broken.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/user-manual.txt

@@ -1357,7 +1357,7 @@ $ gitk --merge
 These will display all commits which exist only on HEAD or on
 MERGE_HEAD, and which touch an unmerged file.
 
-You may also use gitlink:git-mergetool, which lets you merge the
+You may also use gitlink:git-mergetool[1], which lets you merge the
 unmerged files using external tools such as emacs or kdiff3.
 
 Each time you resolve the conflicts in a file and update the index: