Fix document for `Mina_numbers.Global_slot_since_hard_fork`

[glyh]

In that file it says this:

(** See documentation of the {!Mina_wire_types} library *)
module Wire_types = Mina_wire_types.Mina_numbers.Global_slot_since_hard_fork

However no doc is present in Mina_wire_types.Mina_numbers.Global_slot_since_hard_fork