Use three keys in combine_keys example

We can combine two keys with `pk1.combine(&pk2)`, for the `combine_keys`
example we might as well show off the ability to combine more than two
keys.

Author: tcharding

src/key.rs

@@ -540,7 +540,8 @@ impl PublicKey {
     /// let mut rng = rand::thread_rng();
     /// let (_, pk1) = secp.generate_keypair(&mut rng);
     /// let (_, pk2) = secp.generate_keypair(&mut rng);
-    /// let sum = PublicKey::combine_keys(&[&pk1, &pk2]).expect("It's improbable to fail for 2 random public keys");
+    /// let (_, pk3) = secp.generate_keypair(&mut rng);
+    /// let sum = PublicKey::combine_keys(&[&pk1, &pk2, &pk3]).expect("It's improbable to fail for 3 random public keys");
     /// # }
     /// ```
     pub fn combine_keys(keys: &[&PublicKey]) -> Result<PublicKey, Error> {