tc: m_connmark: Fix help text

When specifying a conntrack zone, the 'zone' keyword has to be used
before the actual zone index.
Signed-off-by: Phil Sutter <phil@nwl.cc>

tc/m_connmark.c

@@ -27,7 +27,7 @@
 static void
 explain(void)
 {
-	fprintf(stderr, "Usage: ... connmark [ZONE] [BRANCH] [index <INDEX>]\n");
+	fprintf(stderr, "Usage: ... connmark [zone ZONE] [BRANCH] [index <INDEX>]\n");
 	fprintf(stderr, "where :\n"
 		"\tZONE is the conntrack zone\n"
 		"\tBRANCH := reclassify|pipe|drop|continue|ok\n");