Merge #510: Fix incorrect method call

cd7a6b3 Fix incorrect method call (Tobin C. Harding)

Pull request description:

  We have the following method on `SecretKey`

  ```
      pub fn sign_ecdsa(&self, msg: Message) -> ecdsa::Signature {
          SECP256K1.sign_ecdsa(&msg, self)
      }
  ```

  But we have a method call in rustdocs
  ```
  //! let (secret_key, public_key) = generate_keypair(&mut thread_rng());
  //! let message = Message::from_hashed_data::<sha256::Hash>("Hello World!".as_bytes());
  //!
  //! let sig = secret_key.sign_ecdsa(&message, &secret_key);
  ```

  This is incorrect, I have no idea why this code builds.

  (Also see #508.)

ACKs for top commit:
  apoelstra:
    ACK cd7a6b3

Tree-SHA512: 5f22157798a4e4a21fff946dd930c62c7d82100a8e729309f6383709cb3fefdd935df1b7954fc545f1c2283a46b8c22f61e1c6d4534cb750eb7ab3b5036eedf5

Author: apoelstra

src/lib.rs

@@ -58,15 +58,15 @@
 //! If the "global-context" feature is enabled you have access to an alternate API.
 //!
 //! ```rust
-//! # #[cfg(all(feature="global-context", feature = "std", feature="rand-std", features = "bitcoin-hashes-std"))] {
+//! # #[cfg(all(feature = "global-context", feature = "std", feature = "rand-std", feature = "bitcoin-hashes-std"))] {
 //! use secp256k1::rand::thread_rng;
 //! use secp256k1::{generate_keypair, Message};
 //! use secp256k1::hashes::sha256;
 //!
 //! let (secret_key, public_key) = generate_keypair(&mut thread_rng());
 //! let message = Message::from_hashed_data::<sha256::Hash>("Hello World!".as_bytes());
 //!
-//! let sig = secret_key.sign_ecdsa(&message, &secret_key);
+//! let sig = secret_key.sign_ecdsa(message);
 //! assert!(sig.verify(&message, &public_key).is_ok());
 //! # }
 //! ```