Set the wrap option to false when using Rouge

Since Rouge yields the pre tag with a class attribute but we don't want
it, we should set the wrap parameter to false when instantiating a new
formatter object.

Also use Rouge::Formatter#format instead of #render which is deprecated
and will be removed in the near future.
Fix various typos and improve the doc
Remove a useless given step

Since the highlighter configuration option should default to pygments,
we don't have to explicitly set it in the step testing the output with
pygments.