Fixed Confusing Title in User Form

2 years back, when starting with openerp, it took me quit a while to figure out, that
odoo['Usability'] != english['Usability']

Maybe this string choice was derived from the logic to determine "what can be used", this however is in plain english better represneted by "Activate" or "Activation"

english['Usability'] refers in a wider sens to "User Friendliness".

This wrong usage might have fool millions of users arround the world.