Remove doubled full stop in system/error.php

In system/error.php, we have a full stop (".") after the "contact the system admin" string. Because a full stop is already included in said language string (see [L116](https://github.com/joomla/joomla-cms/blob/staging/language/en-GB/en-GB.ini#L116)), there are two at the final output. 
In Beez and Protostar, this full stop is already eliminated.