(docs) Fixed spelling of available in authentication guide (angular#1068)

Author: Travis Layne Wilson

docs/5-user-authentication.md

@@ -4,7 +4,7 @@
 
 `AngularFireAuth.auth` returns an initialized
 `firebase.auth.Auth` instance, allowing you to log users in, out, etc. [See
-the Firebase docs for more information on what methods are availabile.](https://firebase.google.com/docs/reference/js/firebase.auth.Auth)
+the Firebase docs for more information on what methods are available.](https://firebase.google.com/docs/reference/js/firebase.auth.Auth)
 
 **Example app:**