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 represented 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. Applying this patch: not any more... :+1: