Correctly mark rust code in README.md

Co-authored-by: Alex Martens <alex@thinglab.org>

Author: jannic

README.md

@@ -14,7 +14,7 @@ This project is developed and maintained by the [Cortex-M team][team].
 
 Starting with Rust 1.68, this crate can be used as a global allocator:
 
-```
+```rust
 #![no_std]
 #![no_main]