doc: clapper: Add "app-id" to migrating MPRIS example

It is a new option now required to be set, so migration guide should have it

Author: Rafostar

doc/reference/clapper/migrating-to-010.md

@@ -32,6 +32,7 @@ ClapperEnhancerProxy *proxy = clapper_enhancer_proxy_list_get_proxy_by_module (p
 
 if (proxy) {
   clapper_enhancer_proxy_set_locally (proxy,
+      "app-id", APP_ID,
       "own-name", mpris_name,
       "identity", APP_NAME,
       "desktop-entry", APP_ID, NULL);