merge-tree: fix link formatting in html docs

In the html documentation the link to the "OUTPUT" section is surrounded
by square brackets. Fix this by adding explicit link text to the cross
reference.

Signed-off-by: Phillip Wood <[email protected]>

Author: phillipwood

Documentation/git-merge-tree.txt

@@ -49,7 +49,8 @@ OPTIONS
 	Do not quote filenames in the <Conflicted file info> section,
 	and end each filename with a NUL character rather than
 	newline.  Also begin the messages section with a NUL character
-	instead of a newline.  See <<OUTPUT>> below for more information.
+	instead of a newline.  See <<OUTPUT,OUTPUT>> below for more
+	information.
 
 --name-only::
 	In the Conflicted file info section, instead of writing a list