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.)
Merge pull request #474 from sharplet/contributing-ocd

Reindent numbered lists in CONTRIBUTING.md