Update SlackOAuthV2.php

Fix oauthv2 facade acessor name

#33

Author: vluzrmos

src/Vluzrmos/SlackApi/Facades/SlackOAuthV2.php

@@ -11,6 +11,6 @@ class SlackOAuthV2 extends Facade
      */
     protected static function getFacadeAccessor()
     {
-        return 'slack.oauthv2';
+        return 'slack.o_auth_v2';
     }
 }