fix prefered_route check

(cherry picked from commit 7daecd8dda)

kamailio/dispatcher-role-5.1.cfg

@@ -165,7 +165,7 @@ route[DISPATCHER_PREFERRED_ROUTE]
     ######
     # check if the preferred route is active
     ######
-    if(!ds_is_from_list(-1, 6, "$var(prefered_route"))) {
+    if(!ds_is_from_list(-1, 6, "$var(prefered_route)")) {
        xlog("L_INFO", "$ci|log|associated media server $var(prefered_route) is inactive, moving to $avp(ds_dst)\n");
        return -1;
     }