docs(ionic): add AngularFireDatabase in the import section (#1242)

Author: adnan333bd

docs/ionic/v3.md

@@ -195,7 +195,7 @@ import { MyApp } from './app.component';
 import { HomePage } from '../pages/home/home';
 
 import { AngularFireModule } from 'angularfire2';
-import { AngularFireDatabaseModule } from 'angularfire2/database';
+import { AngularFireDatabaseModule, AngularFireDatabase } from 'angularfire2/database';
 import { AngularFireAuthModule } from 'angularfire2/auth';
 
 export const firebaseConfig = {