Merge #708: scalar: fix PrivateKey->SecretKey in docstring

37d040c scalar: fix PrivateKey->SecretKey in docstring (Marko Bencun)

Pull request description:

ACKs for top commit:
  Kixunil:
    ACK 37d040c
  tcharding:
    ACK 37d040c

Tree-SHA512: 26ea05cf555463387ea545ef9e81f1ad4e0689fe70d5b4256be45e3ae765b7b23f1c10ad6d14ee80a561fd2481ad8d30fe0c2ac440bbd6f33bfee91e5441d93d

Author: apoelstra

src/scalar.rs

@@ -14,7 +14,7 @@ use crate::constants;
 
 /// Positive 256-bit integer guaranteed to be less than the secp256k1 curve order.
 ///
-/// The difference between `PrivateKey` and `Scalar` is that `Scalar` doesn't guarantee being
+/// The difference between `SecretKey` and `Scalar` is that `Scalar` doesn't guarantee being
 /// securely usable as a private key.
 ///
 /// **Warning: the operations on this type are NOT constant time!**