Update lib.rs

Removed duplicate words

src/libstd/lib.rs

@@ -44,7 +44,7 @@
 //! lives in the [`vec`](vec/index.html) module. Contiguous, unsized regions
 //! of memory, `[T]`, commonly called "slices", and their borrowed versions,
 //! `&[T]`, commonly called "borrowed slices", are built-in types for which the
-//! for which the [`slice`](slice/index.html) module defines many methods.
+//! [`slice`](slice/index.html) module defines many methods.
 //!
 //! `&str`, a UTF-8 string, is a built-in type, and the standard library
 //! defines methods for it on a variety of traits in the