Fix formatting and improve samples/README.md
Fix formatting and improve samples/README.md

There was an error with the names of two CubicPilot achievements, otherwise this is mostly formatting improvements and some minor text changes.
claywilkinson/play-games-plugin-for-unity,claywilkinson/play-games-plugin-for-unity,claywilkinson/play-games-plugin-for-unity,claywilkinson/play-games-plugin-for-unity