Fix README license badge URL

Update the README license badge to use the generic 'mit' image URL instead of the crate-specific path (changes badge src from `ph-curves` to `mit`) so the MIT license displays correctly.

Author: swgiacomelli

README.md

@@ -3,7 +3,7 @@
 [![Crates.io](https://img.shields.io/crates/v/ph-curves)](https://crates.io/crates/ph-curves)
 [![docs.rs](https://img.shields.io/docsrs/ph-curves)](https://docs.rs/ph-curves)
 [![CI](https://github.com/photon-circus/ph-curves/actions/workflows/ci.yml/badge.svg)](https://github.com/photon-circus/ph-curves/actions/workflows/ci.yml)
-[![License: MIT](https://img.shields.io/crates/l/ph-curves)](LICENSE.md)
+[![License: MIT](https://img.shields.io/crates/l/mit)](LICENSE.md)
 [![MSRV](https://img.shields.io/badge/MSRV-1.92.0-blue)](rust-toolchain.toml)
 [![no_std](https://img.shields.io/badge/no__std-yes-green)](src/lib.rs)