fix the RST syntax of config.sample.php

With this the latest fixes for the config.sample.php -> doc generation are fixed.

cc @LukasReschke @wakeup @carlaschroder 

The result is already in documentation repo and provide a working syntax of the RST file: owncloud/documentation@12f2b5cb249a48003b7af788bcdcc7de4535dfbe

@karlitschek This also needs a backport to stable7