fix shortcuts of Alt-Ergo 2.5.x
As reported on the Why3 Club, the new versions 2.5.x of Alt-Ergo are now recognized with shortcuts "alt-ergo-smt" and "alt-ergo-bv", and not anymore by "alt-ergo".
The ocamldoc documentation of Whyconf
API about filter provers should be made clearer. It is not even clear whether it should support shortcuts.