[docs] Fix issue #8288695: Typo in OAuth2 training

There's a typo in the "Authenticating to OAuth2 Services" training
class, under "Request an Auth Token... Again" (#RequestAgain).

The call to result.get() will fail, because AccountManagerFuture
doesn't support this method. Instead, getResult() needs to be
called first.

Change-Id: Ia25b9364cd14180cf7ddac50057a2ec03e01ed1f

docs/html/training/id-auth/authenticate.jd

@@ -179,7 +179,7 @@ private class OnTokenAcquired implements AccountManagerCallback<Bundle> {
     @Override
     public void run(AccountManagerFuture<Bundle> result) {
         ...
-        Intent launch = (Intent) result.get(AccountManager.KEY_INTENT);
+        Intent launch = (Intent) result.getResult().get(AccountManager.KEY_INTENT);
         if (launch != null) {
             startActivityForResult(launch, 0);
             return;