fix wrong file paths in uninstall

fixes an error on uninstall:

<pre>
Warning: require_once(/home/paulwp/spine.paulwp.com/wp-content/plugins/backupwordpress//classes/services.php) [function.require-once]: failed to open stream: No such file or directory in /home/paulwp/spine.paulwp.com/wp-content/plugins/backupwordpress/uninstall.php on line 11

Fatal error: require_once() [function.require]: Failed opening required '/home/paulwp/spine.paulwp.com/wp-content/plugins/backupwordpress//classes/services.php' (include_path='.:/usr/local/lib/php:/usr/local/php5/lib/pear') in /home/paulwp/spine.paulwp.com/wp-content/plugins/backupwordpress/uninstall.php on line 11
</pre>