ch15-02-deref: Fix explanation on immut-to-mut

On a hypothetical immut-to-mut deref, the resulting mut ref should be
unique. The aliasing rules say the mutable reference should be the only
reference, so correct the language accordingly.

Signed-off-by: Eddy Petrișor <eddy.petrisor@gmail.com>

src/ch15-02-deref.md

@@ -376,6 +376,6 @@ a mutable reference, that mutable reference must be the only reference to that
 data (otherwise, the program wouldn’t compile). Converting one mutable
 reference to one immutable reference will never break the borrowing rules.
 Converting an immutable reference to a mutable reference would require that
-there is only one immutable reference to that data, and the borrowing rules
+there is only one mutable reference to that data, and the borrowing rules
 don’t guarantee that. Therefore, Rust can’t make the assumption that converting
 an immutable reference to a mutable reference is possible.