Improve lib.rs rustdocs

Improve the main docs by doing:

- Remove unneeded `self` from use statement
- Add code ticks to `bitcoin_hashes`

src/lib.rs

@@ -57,13 +57,13 @@
 //! Alternately, keys and messages can be parsed from slices, like
 //!
 //! ```rust
-//! use self::secp256k1::{Secp256k1, Message, SecretKey, PublicKey};
+//! use secp256k1::{Secp256k1, Message, SecretKey, PublicKey};
 //!
 //! let secp = Secp256k1::new();
 //! let secret_key = SecretKey::from_slice(&[0xcd; 32]).expect("32 bytes, within curve order");
 //! let public_key = PublicKey::from_secret_key(&secp, &secret_key);
 //! // This is unsafe unless the supplied byte slice is the output of a cryptographic hash function.
-//! // See the above example for how to use this library together with bitcoin_hashes.
+//! // See the above example for how to use this library together with `bitcoin_hashes`.
 //! let message = Message::from_slice(&[0xab; 32]).expect("32 bytes");
 //!
 //! let sig = secp.sign_ecdsa(&message, &secret_key);