README: change shield colors to improve contrast

Author: newAM

README.md

@@ -4,8 +4,8 @@
 [![stable-docs](https://img.shields.io/badge/docs-stable-blue)](https://docs.rs/stm32wlxx-hal/)
 [![nightly-docs](https://img.shields.io/badge/docs-nightly-blue)](https://stm32-rs.github.io/stm32wlxx-hal/stm32wlxx_hal/index.html)
 [![crates.io](https://img.shields.io/crates/v/stm32wlxx-hal.svg)](https://crates.io/crates/stm32wlxx-hal)
-![license](https://img.shields.io/crates/l/stm32wlxx-hal?color=green)
-![MSRV](https://img.shields.io/badge/MSRV-1.56-informational)
+![license](https://img.shields.io/crates/l/stm32wlxx-hal?color=black)
+![MSRV](https://img.shields.io/badge/MSRV-1.56-informational?color=purple)
 
 Embedded rust HAL (hardware abstraction layer) for the STM32WL series.