documentation fix: git difftool uses diff tools, not merge tools.

Let the documentation for -t list valid *diff* tools,
not valid *merge* tools.
Signed-off-by: NThomas Hochstein <thh@inter.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-difftool.txt

@@ -31,7 +31,7 @@ OPTIONS
 -t <tool>::
 --tool=<tool>::
 	Use the diff tool specified by <tool>.
-	Valid merge tools are:
+	Valid diff tools are:
 	araxis, bc3, diffuse, emerge, ecmerge, gvimdiff, kdiff3,
 	kompare, meld, opendiff, p4merge, tkdiff, vimdiff and xxdiff.
 +