Fix dead link for std::sync::mpsc.

src/libstd/lib.rs

@@ -75,7 +75,7 @@
 //!
 //! The [`thread`](thread/index.html) module contains Rust's threading abstractions.
 //! [`sync`](sync/index.html) contains further, primitive, shared memory types,
-//! including [`atomic`](sync/atomic/index.html), and [`mpsc`](sync/mpmc/index.html),
+//! including [`atomic`](sync/atomic/index.html), and [`mpsc`](sync/mpsc/index.html),
 //! which contains the channel types for message passing.
 //!
 //! Common types of I/O, including files, TCP, UDP, pipes, Unix domain sockets,