Fix comment about max number of events (#488)

Multicast message can send up to 500 messages, not 100

https://github.com/firebase/firebase-admin-java/blob/408b85f7d0072e8fb3b462aa2317843d683e7ad7/src/main/java/com/google/firebase/messaging/MulticastMessage.java#L46

Author: rlazo

src/test/java/com/google/firebase/snippets/FirebaseMessagingSnippets.java

@@ -151,7 +151,7 @@ public void sendAll() throws FirebaseMessagingException {
 
   public void sendMulticast() throws FirebaseMessagingException {
     // [START send_multicast]
-    // Create a list containing up to 100 registration tokens.
+    // Create a list containing up to 500 registration tokens.
     // These registration tokens come from the client FCM SDKs.
     List<String> registrationTokens = Arrays.asList(
         "YOUR_REGISTRATION_TOKEN_1",