fix Sphinx error: add empty line after code-block

The "minimal site configuration on Ubuntu 12.04" is currently not visible at [1].
What's missing is the blank line after the Sphinx directive.

[1] http://doc.owncloud.org/server/6.0/admin_manual/installation/installation_source.html
Merge pull request #285 from rol/patch-1

fix Sphinx error: add empty line after code-block