README: Use correct license

Currently the readme badge is displaying Apache, we are not using the
Apache license, it should be CC0.

Use the correct CC0-1.0 Universal license.

Author: tcharding

README.md

@@ -9,7 +9,7 @@
 
   <p>
     <a href="https://crates.io/crates/bitcoin"><img alt="Crate Info" src="https://img.shields.io/crates/v/bitcoin.svg"/></a>
-    <a href="https://github.com/rust-bitcoin/rust-bitcoin/blob/master/LICENSE"><img alt="MIT or Apache-2.0 Licensed" src="https://img.shields.io/badge/license-MIT%2FApache--2.0-blue.svg"/></a>
+    <a href="https://github.com/rust-bitcoin/rust-bitcoin/blob/master/LICENSE"><img alt="CC0 1.0 Universal Licensed" src="https://img.shields.io/badge/license-CC0--1.0-blue.svg"/></a>
     <a href="https://github.com/rust-bitcoin/rust-bitcoin/actions?query=workflow%3AContinuous%20integration"><img alt="CI Status" src="https://github.com/rust-bitcoin/rust-bitcoin/workflows/Continuous%20integration/badge.svg"></a>
     <a href="https://docs.rs/bitcoin"><img alt="API Docs" src="https://img.shields.io/badge/docs.rs-bitcoin-green"/></a>
     <a href="https://blog.rust-lang.org/2020/02/27/Rust-1.41.1.html"><img alt="Rustc Version 1.41.1+" src="https://img.shields.io/badge/rustc-1.41.1%2B-lightgrey.svg"/></a>