Correct counting to four in cell module docs

Author: TomFryersMidsummer

core/src/cell.rs

@@ -22,8 +22,8 @@
 //! (mutable via `&T`), in contrast with typical Rust types that exhibit 'inherited mutability'
 //! (mutable only via `&mut T`).
 //!
-//! Cell types come in three flavors: `Cell<T>`, `RefCell<T>`, and `OnceCell<T>`. Each provides
-//! a different way of providing safe interior mutability.
+//! Cell types come in four flavors: `Cell<T>`, `RefCell<T>`, `OnceCell<T>`, and `LazyCell<T>`.
+//! Each provides a different way of providing safe interior mutability.
 //!
 //! ## `Cell<T>`
 //!