Fix a typo (#397)

src/lib.rs

@@ -16,7 +16,7 @@
 //!
 //! Ureq is in pure Rust for safety and ease of understanding. It avoids using
 //! `unsafe` directly. It [uses blocking I/O][blocking] instead of async I/O, because that keeps
-//! the API simple and and keeps dependencies to a minimum. For TLS, ureq uses
+//! the API simple and keeps dependencies to a minimum. For TLS, ureq uses
 //! [rustls].
 //!
 //! Version 2.0.0 was released recently and changed some APIs. See the [changelog] for details.