Center aligning code blocks for appium.io and added instructions

- Added center to code snippets to make it center aligned for appium.io
- Added instructions about center aligning code in appium.io
https://gist.githubusercontent.com/bootstraponline/d1d0eb70090c1cef987d/raw/f8ff027793697300c2185cd8fa8c0c6f716ccb42/fix.md