Fix formatting of samples/README.md
Fix formatting of samples/README.md

Add a few missing br elements from formatting commit 7344692f49ccd3187c6944e47441eb79a0b03a83
claywilkinson/play-games-plugin-for-unity,claywilkinson/play-games-plugin-for-unity,claywilkinson/play-games-plugin-for-unity,claywilkinson/play-games-plugin-for-unity