Fix example commands on the index page

This basically reverts the modifications made on #51 (5591b2d), as at
the time it wasn't possible to call the `bootstrap-vz` command from a
parent folder.

I've also changed the command prompt sign from `$` to `#` where the
user is `root`.
Merge pull request #61 from myhro/doc-index

Fix example commands on the index page