Fixed the package name to conform to standard in the repo

- replaced com.example with github.nisrulz

using_firebase_db/android/app/build.gradle

@@ -33,7 +33,7 @@ android {
 
     defaultConfig {
         // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
-        applicationId "com.example.using_firebase_db"
+        applicationId "github.nisrulz.using_firebase_db"
         minSdkVersion 16
         targetSdkVersion 28
         versionCode flutterVersionCode.toInteger()