Minor, fix the scoping of a static variable

Auditors: chad, btrahan

src/applications/auth/provider/PhabricatorAuthProviderOAuthFacebook.php

@@ -122,7 +122,7 @@ final class PhabricatorAuthProviderOAuthFacebook
     }
 
     return $fb_provider->getProviderConfig()->getProperty(
-      PhabricatorAuthProviderOAuth::PROPERTY_APP_ID);
+      self::PROPERTY_APP_ID);
   }
 
 }