settings.js: fix apparent typo in SettingsManager.unregister

Removing my Alt-Tab-Enhanced applet from the panel produces the following error output:
````
   917	error t=2014-01-27T19:06:42.690Z this.instances is undefined
   918	trace t=2014-01-27T19:06:42.691Z 
   919	<----------------
   920	("Alt_Tab_Enhanced@autarkper","Alt_Tab_Enhanced@autarkper")@/usr/share/cinnamon/js/ui/settings.js:817
   921	()@/usr/share/cinnamon/js/ui/settings.js:521
   922	disable()@/home/pang/.local/share/cinnamon/applets/Alt_Tab_Enhanced@autarkper/applet.js:2958
   923	()@/home/pang/.local/share/cinnamon/applets/Alt_Tab_Enhanced@autarkper/applet.js:3205
   924	()@/usr/share/cinnamon/js/ui/applet.js:332
   925	removeAppletFromPanels([object Object])@/usr/share/cinnamon/js/ui/appletManager.js:211
   926	onEnabledAppletsChanged([object _private_Gio_Settings],"enabled-applets")@/usr/share/cinnamon/js/ui/appletManager.js:168
   927	---------------->
   928	error t=2014-01-27T19:06:42.691Z Error during on_applet_removed_from_panel() call on applet: Alt_Tab_Enhanced@autarkper/454
````

As far as I can tell, this is caused by an apparent typo introduced in commit 26e9b3dd: "this.instances" is only used once, and should most likely be "this.uuids"; by changing to the latter I can make the error go away.