docs(getting-started): /compat missing from firebase import (#2917)

Fix for `/compat` missing from firebase import

Author: syamanashi

docs/auth/getting-started.md

@@ -11,7 +11,7 @@ the Firebase docs for more information on what methods are available.](https://f
 ```ts
 import { Component } from '@angular/core';
 import { AngularFireAuth } from '@angular/fire/compat/auth';
-import firebase from 'firebase/app';
+import firebase from 'firebase/compat/app';
 
 @Component({
   selector: 'app-root',