Reindent numbered lists in CONTRIBUTING.md

Markdown not only requires multi-paragraph list items to be indented at the same level, but explicitly requires either 1 tab or 4 spaces ([Markdown Syntax#list](http://daringfireball.net/projects/markdown/syntax#list)).

Reindent 3 -> 4 spaces, as the 3-space indent was throwing off Marked.app (the second paragraph in the list item was being interpreted as at the top level). (I believe Marked.app uses MultiMarkdown out of the box.)