mergetool--lib: Fix typo in the merge/difftool help

The help text for the `tool` flag should mention:

    --tool=<tool>

instead of:

    --tool-<tool>
Signed-off-by: NStefan Saasen <ssaasen@atlassian.com>
Reviewed-by: NDavid Aguilar <davvid@gmail.com>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>

git-mergetool--lib.sh

@@ -264,7 +264,7 @@ list_merge_tool_candidates () {
 }
 
 show_tool_help () {
-	tool_opt="'git ${TOOL_MODE}tool --tool-<tool>'"
+	tool_opt="'git ${TOOL_MODE}tool --tool=<tool>'"
 
 	tab='	'
 	LF='