Fix minor typo in std::str module docs

Author: Valloric

src/libstd/str.rs

@@ -16,7 +16,7 @@ String manipulation
 
 Rust's string type is one of the core primitive types of the language. While
 represented by the name `str`, the name `str` is not actually a valid type in
-Rust. Each string must also be decorated with how its ownership. This means that
+Rust. Each string must also be decorated with its ownership. This means that
 there are three common kinds of strings in rust:
 
 * `~str` - This is an owned string. This type obeys all of the normal semantics