Fix markdown in contribute page

The markdown renderer was interpreting the tab as the start of a code span.

This is visible on the [live page](http://ceylon-lang.org/code/contribute/), but not on the [Github version](https://github.com/ceylon/ceylon-lang.org/blob/b46a0213b117679e4f3b9af402082a06a461e65e/code/contribute.md), because they use different markdown renderers.

CC #289.

Also fixes some other very minor things.