doc: git-gui browser does not default to HEAD

37cd4f7 ("Document git-gui, git-citool as mainporcelain manual pages",
2007-06-21) documented the default, but was shortly followed by c52c945
("git-gui: Allow blame/browser subcommands on bare repositories",
2007-07-17) which, it would appear, as a side effect, removed that default.

Finally document that change.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Reviewed-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-gui.txt

@@ -35,7 +35,7 @@ blame::
 
 browser::
 	Start a tree browser showing all files in the specified
-	commit (or 'HEAD' by default).  Files selected through the
+	commit.  Files selected through the
 	browser are opened in the blame viewer.
 
 citool::