registrar: fix expires value var name

kamailio/registrar-role.cfg

@@ -145,10 +145,10 @@ route[ATTEMPT_AUTHORIZATION]
     if($var(expires) == 0) {
         xlog("L_INFO", "$ci|end|unregister request from $Au $si:$sp");
         exit;
-    } else if($var(Expires) < REGISTRAR_MIN_EXPIRES) {
-        $var(Expires) = REGISTRAR_MIN_EXPIRES;
-    } else if($var(Expires) > REGISTRAR_MAX_EXPIRES) {
-        $var(Expires) = REGISTRAR_MAX_EXPIRES;
+    } else if($var(expires) < REGISTRAR_MIN_EXPIRES) {
+        $var(expires) = REGISTRAR_MIN_EXPIRES;
+    } else if($var(expires) > REGISTRAR_MAX_EXPIRES) {
+        $var(expires) = REGISTRAR_MAX_EXPIRES;
     }
 
     ## Fix for wss websockets.