docs(app, android): suggest 4.3.8 of google-play-services plugin

4.3.6 was non-functional, code was updated but docs weren't
Credit to @marqroldan for spotting it, thanks!

Author: mikehardy

docs/index.md

@@ -61,7 +61,7 @@ First, add the `google-services` plugin as a dependency inside of your `/android
 buildscript {
   dependencies {
     // ... other dependencies
-    classpath 'com.google.gms:google-services:4.3.6'
+    classpath 'com.google.gms:google-services:4.3.8'
     // Add me --- /\
   }
 }