typo

src/libcore/cell.rs

@@ -11,7 +11,7 @@
 //! mutate it.
 //!
 //! Shareable mutable containers exist to permit mutability in a controlled manner, even in the
-//! presence of aliasing. Both `Cell<T>` and `RefCell<T>` allows to do this in a single threaded
+//! presence of aliasing. Both `Cell<T>` and `RefCell<T>` allow doing this in a single-threaded
 //! way. However, neither `Cell<T>` nor `RefCell<T>` are thread safe (they do not implement
 //! `Sync`). If you need to do aliasing and mutation between multiple threads it is possible to
 //! use [`Mutex`](../../std/sync/struct.Mutex.html),