Fixed the package name to conform to standard in the repo

- replaced com.example with github.nisrulz

using_firebase_db/android/app/src/debug/AndroidManifest.xml

@@ -1,5 +1,5 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
-    package="com.example.using_firebase_db">
+    package="github.nisrulz.using_firebase_db">
     <!-- Flutter needs it to communicate with the running application
          to allow setting breakpoints, to provide hot reload, etc.
     -->