s/url.purge/purge.url/ in help

The documentation referred to deprecated url.purge, make it point to
purge.url instead.  Maybe it should be updated to the new purge
syntax.

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4397 d4fa192b-c00b-0410-8231-f00ffab90ce4

bin/varnishd/mgt_param.c

@@ -483,7 +483,7 @@ static const struct parspec input_parspec[] = {
 		"Objects already cached will not be affected by changes "
 		"made until they are fetched from the backend again.\n"
 		"To force an immediate effect at the expense of a total "
-		"flush of the cache use \"url.purge .\"",
+		"flush of the cache use \"purge.url .\"",
 		0,
 		"120", "seconds" },
 	{ "sess_workspace", tweak_uint, &master.sess_workspace, 1024, UINT_MAX,