Remove extern crate from example

Author: djc

src/lib.rs

@@ -22,7 +22,6 @@
 //! ```rust
 //! // This could be a cargo build script
 //!
-//! extern crate rustc_version;
 //! use rustc_version::{version, version_meta, Channel, Version};
 //!
 //! fn main() {