Fix method visibility for PhabricatorBitbucketAuthProvider::getProviderConfigurationHelp()
Summary: Ref T6822.
Test Plan: Visual inspection. This method is only called from within PhabricatorOAuthAuthProvider subclasses.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T6822
Differential Revision: https://secure.phabricator.com/D11246