disambiguate sentence

The phrase 'may not' could mean 'might not' or 'is not allowed to'.

src/types/closure.md

@@ -106,7 +106,7 @@ let z = &x;
 
 In this case, borrowing `x` mutably is not possible, because `x` is not `mut`.
 But at the same time, borrowing `x` immutably would make the assignment illegal,
-because a `& &mut` reference may not be unique, so it cannot safely be used to
+because a `& &mut` reference might not be unique, so it cannot safely be used to
 modify a value. So a unique immutable borrow is used: it borrows `x` immutably,
 but like a mutable borrow, it must be unique. In the above example, uncommenting
 the declaration of `y` will produce an error because it would violate the