[SPARK-2735][Documentation] Remove deprecation warning in 'jekyll build' for pygments

The following warning was corrected which shows up when one does 'jekyll build' in the docs folder
    "Deprecation: The 'pygments' configuration option has been renamed to 'highlighter'. Please update your config file accordingly. The allowed values are 'rouge', 'pygments' or null."

    Reference:https://github.com/mmistakes/hpstr-jekyll-theme/issues/25