Fixed a typo that is annyoing in the README

Changed  (PlayGamesPlatform) Social.Active).ShowLeaderboardUI("Cfji293fjsie_QA"); to 
 ((PlayGamesPlatform) Social.Active).ShowLeaderboardUI("Cfji293fjsie_QA");
Merge pull request #38 from DavidSkeppstedt/patch-1

Fixed a typo that is annyoing in the README