docs(Auth-with-Ionic3-Angular4) Fix Providers (#1159)

* Add 'AngularFireDatabase' into providers

AngularFireDatabase should be declared into providers section in app.module in order to be used by components

* Update auth-with-ionic3-angular4.md instructions

Update instructions so that they reflect the code below

Author: SpiralOutDotEu

docs/Auth-with-Ionic3-Angular4.md

@@ -182,9 +182,9 @@ and add the following three entries.
 
 >2) Define your firebaseConfig constant.
 
->3) Initialize your app, by adding AngularFireModule in the "imports" array in @NgModule
+>3) Initialize your app, by adding AngularFireModule and AngularFireAuthModule in the "imports" array in @NgModule
 
->3) Also, add AngularFireDatabaseModule in the "imports" array in @NgModule
+>4) Also, add AngularFireDatabase in the "providers" array in @NgModule
 
 your `app.module.ts` file should look something like this.
 
@@ -231,6 +231,7 @@ export const firebaseConfig = {
   providers: [
     StatusBar,
     SplashScreen,
+    AngularFireDatabase,
     {provide: ErrorHandler, useClass: IonicErrorHandler}
   ]
 })