Fix broken TLS anchor link in docs overview

Now correctly links to:
https://docs.rs/ureq/latest/ureq/#https--tls--ssl

Introduced in 56276c3742.

Also fixes a rustdoc typo spotted by my IDE's spell checker whilst the
project was open.

src/lib.rs

@@ -17,7 +17,7 @@
 //! 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
-//! [rustls or native-tls](#tls).
+//! [rustls or native-tls](#https--tls--ssl).
 //!
 //! Version 2.0.0 was released recently and changed some APIs. See the [changelog] for details.
 //!