Remove one instance of Secp256k1::new from docs

One stale construction of a context in docs, remove it.

Author: tcharding

src/secret.rs

@@ -131,9 +131,8 @@ impl Keypair {
     /// ```
     /// # #[cfg(feature = "std")] {
     /// # use std::str::FromStr;
-    /// use secp256k1::{Keypair, Secp256k1, SecretKey};
+    /// use secp256k1::{Keypair, SecretKey};
     ///
-    /// let secp = Secp256k1::new();
     /// let key = SecretKey::from_str("0000000000000000000000000000000000000000000000000000000000000001").unwrap();
     /// let key = Keypair::from_secret_key(&key);
     /// // Here we explicitly display the secret value: