Fix wrong example in `lib.rs`
The example code at `lib.rs` is repeating the assertion of `child_xprv_str`, while the `child_xpub_str` remains unused/untested.
//!
//! // Serialize `child_xpub` as a string with the `xpub` prefix.
//! let child_xpub_str = child_xpub.to_string(Prefix::XPUB);
-//! assert!(child_xprv_str.starts_with("xprv"));
+//! assert!(child_xpub_str.starts_with("xpub"));
//!
//! // Get the ECDSA/secp256k1 signing and verification keys for the xprv and xpub
//! let signing_key = child_xprv.private_key();
