[Minor] Increase RDNS_NONE score to 2.0

conf/scores.d/headers_group.conf

@@ -51,7 +51,7 @@ symbols = {
         description = "One received header in a message";
     }
     "RDNS_NONE" {
-        weight = 1.0;
+        weight = 2.0;
         description = "Cannot resolve reverse DNS for sender's IP";
     }
     "RDNS_DNSFAIL" {