Fix Rust linting error

src/libcollections/str.rs

@@ -10,8 +10,9 @@
 
 //! Unicode string slices.
 //!
-//! The `&str` type is one of the two main string types, the other being `String`. Unlike its `String` counterpart, its contents
-//! are borrowed and therefore cannot be moved someplace else.
+//! The `&str` type is one of the two main string types, the other being `String`.
+//! Unlike its `String` counterpart, its contents are borrowed and therefore
+//! cannot be moved someplace else.
 //!
 //! # Basic Usage
 //! A basic string declaration of `&str` type:
@@ -22,8 +23,8 @@
 //!
 //! Here we have declared a string literal, also known as a string slice.
 //! String literals have a static lifetime, which means the string `hello_world`
-//! is guaranteed to be valid for the duration of the entire program. We can explicitly specify
-//! `hello_world`'s lifetime as well:
+//! is guaranteed to be valid for the duration of the entire program.
+//! We can explicitly specify `hello_world`'s lifetime as well:
 //!
 //! ```
 //! let hello_world:&'static str = "Hello, world!";