fix(messaging): types for deleteToken() (#1890)

Author: icetortoise

src/messaging/messaging.ts

@@ -14,7 +14,7 @@ export class AngularFireMessaging {
   tokenChanges: Observable<string|null>;
   messages: Observable<{}>;
   requestToken: Observable<string|null>;
-  deleteToken: (string) => Observable<boolean>;
+  deleteToken: (token: string) => Observable<boolean>;
 
   constructor(
     @Inject(FirebaseOptionsToken) options:FirebaseOptions,
@@ -78,4 +78,4 @@ export class AngularFireMessaging {
     );
   }
 
-}
+}