Fix dead link (s/task/thread/)

src/libstd/lib.rs

@@ -73,7 +73,7 @@
 //!
 //! ## Concurrency, I/O, and the runtime
 //!
-//! The [`task`](task/index.html) module contains Rust's threading abstractions,
+//! The [`thread`](thread/index.html) module contains Rust's threading abstractions,
 //! while [`comm`](comm/index.html) contains the channel types for message
 //! passing. [`sync`](sync/index.html) contains further, primitive, shared
 //! memory types, including [`atomics`](sync/atomics/index.html).