contrib/vagrant: fix typo in README

This was introduced in be238b8caa19.
contrib/vagrant: fix extra quotes in code block
contrib/vagrant: make code a real code block

In lists, code blocks must be inteded by 8 spaces.
contrib/vagrant: correctly nest code block in list item
contrib/vagrant: use real link instead of literal text for link
Merge pull request #118 from mgeisler/fix-formatting

Fix README formatting