Fix preference methods returning all the preferences

[paulijar]

Fix what appears to be a copy-paste error:

Calling the method user_preference was returning all the user preferences by invoking the action user_preferences instead of invoking the action matching the method name and returning only the requested preference.

Similarly, the method system_preference was invoking the action system_preferences.

[lachlan-00]

that's one annoying thing about plurals