Fix typo in docs: usize is unsigned, not signed.

Author: dato

src/libstd/primitive_docs.rs

@@ -411,7 +411,7 @@ mod prim_isize { }
 
 #[doc(primitive = "usize")]
 //
-/// The pointer-sized signed integer type.
+/// The pointer-sized unsigned integer type.
 ///
 /// *[See also the `std::usize` module](usize/index.html).*
 ///