chore(docs): Add params to custom RouterStateSerializer example (#589)

docs/router-store/api.md

@@ -104,6 +104,7 @@ import {
 
 export interface RouterStateUrl {
   url: string;
+  params: Params;
   queryParams: Params;
 }
 
@@ -113,12 +114,18 @@ export interface State {
 
 export class CustomSerializer implements RouterStateSerializer<RouterStateUrl> {
   serialize(routerState: RouterStateSnapshot): RouterStateUrl {
+    let route = routerState.root;
+    while (route.firstChild) {
+      route = route.firstChild;
+    }
+
     const { url } = routerState;
     const queryParams = routerState.root.queryParams;
+    const params = route.params;
 
-    // Only return an object including the URL and query params
+    // Only return an object including the URL, params and query params
     // instead of the entire snapshot
-    return { url, queryParams };
+    return { url, params, queryParams };
   }
 }