Minor grammar fixes

I've changed it to 
```
(* List of normalizations to perform : if the type [p_step] had a constructor
   that indicated visiting both left and right branches, we would be able to
   restrict ourselves to the case of only one normalization by hypothesis.
   And since all hypothesis are useful (otherwise they wouldn't be included),
   we would be able to replace [h_step] by a simple list. *)
```