Improve interior mutability slide (#1683)

Mutex is probably more broadly understood by people coming from other
languages.

Fixes #1512.

src/concurrency/shared_state/mutex.md

@@ -1,7 +1,8 @@
 # `Mutex`
 
 [`Mutex<T>`][1] ensures mutual exclusion _and_ allows mutable access to `T`
-behind a read-only interface:
+behind a read-only interface (another form of
+[interior mutability](../../borrowing/interior-mutability)):
 
 ```rust,editable
 use std::sync::Mutex;