Fix link to embassy-usb

Author: jannic

README.md

@@ -42,4 +42,4 @@ Others
 ------
 
 Other implementations for USB in Rust
-* [embassy-usb](https://github.com/embassy-rs/embassy/blob/master/embassy-usb/src/driver.rs), an async variant.
+* [embassy-usb](https://github.com/embassy-rs/embassy/tree/master/embassy-usb) an async variant.