Update README.md

Author: rudib

README.md

@@ -3,7 +3,7 @@ Bit-level packing and unpacking for Rust
 
 [![Crates.io][crates-badge]][crates-url]
 [![Documentation](https://docs.rs/packed_struct/badge.svg)](https://docs.rs/packed_struct)
-![Rust](https://github.com/hashmismatch/packed_struct.rs/workflows/Rust/badge.svg)
+![master](https://github.com/hashmismatch/packed_struct.rs/workflows/Rust/badge.svg)
 
 [crates-badge]: https://img.shields.io/crates/v/packed_struct.svg
 [crates-url]: https://crates.io/crates/packed_struct