fix(Flutter): ':' to '.' in Google services plugin

Author: tyllark

src/fragments/lib/push-notifications/common/getting_started/cross-platform-prereq.mdx

@@ -49,7 +49,7 @@ If you prefer using the plugins DSL, you should add the `plugins` block to the v
 The Firebase documentation directs you to add the Google services plugin to your app `settings.gradle` using the [Gradle plugins DSL](https://docs.gradle.org/current/userguide/plugins.html#sec:plugins_block). However, we recommend you continue to use the [Legacy Plugin Application](https://docs.gradle.org/current/userguide/plugins.html#sec:old_plugin_application) instead:
 
 ```groovy
-id "com.google.gms:google-services" version "4.3.14" apply false
+id "com.google.gms.google-services" version "4.3.14" apply false
 ```
 
 </InlineFilter>