Changed README.md to fix problems with showLeaderboard and signout

[Samleo8]

There are 2 misprints in the README.md:

1. Sign Out

googleplaygame.signout(); should be googleplaygame.signOut();

2. Show Leaderboards

var data = { leaderboardId: "board1" }; googleplaygame.showLeaderboard(leaderboardId);

should be

var data = { leaderboardId: "board1" }; googleplaygame.showLeaderboard(data);