Fixed typo on page 72

[MysaaJava]

U4 was replaced by U3 on page 72, ch3, section 3.2 :

In an empty context, the term Prop has type Type (for any value of the index) :
Check Prop : Type.
Prop : Type@{U3} (* Prop < U3 *)
Set happens to be a name for the smallest element of the hierarchy of Type :
Check Set : Type.
Set : Type@{U4} (* Set < U3 *)

I rewrote U3 with U4.
(i hope i am right !)