Please the make tidy

src/libcore/cell.rs

@@ -24,7 +24,8 @@
 //! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
 //! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked
 //! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt
-//! to borrow a value that is already mutably borrowed; when this happens it results in thread panic.
+//! to borrow a value that is already mutably borrowed; when this happens it results in thread
+//! panic.
 //!
 //! # When to choose interior mutability
 //!