mergetools: vimdiff: fix comment

The name of the variable is wrong, and it can be set to anything, like
1.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Reviewed-by: Fernando Ramos <greenfoo@u92.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mergetools/vimdiff

@@ -29,8 +29,8 @@
 ################################################################################
 
 debug_print () {
-	# Send message to stderr if global variable GIT_MERGETOOL_VIMDIFF is set
-	# to "true"
+	# Send message to stderr if global variable GIT_MERGETOOL_VIMDIFF_DEBUG
+	# is set.
 
 	if test -n "$GIT_MERGETOOL_VIMDIFF_DEBUG"
 	then