UI: Use term 'Preferences' instead of 'User Prefs'

'Preferences' is the term used elsewhere in Blender so this commit makes 
the option more consistent.

In the future, the "default" target could be changed to something more 
descriptive.

release/scripts/startup/bl_operators/userpref.py

@@ -590,7 +590,7 @@ class PREFERENCES_OT_addon_install(Operator):
         name="Target Path",
         items=(
             ('DEFAULT', "Default", ""),
-            ('PREFS', "User Prefs", ""),
+            ('PREFS', "Preferences", ""),
         ),
     )