[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