docs: fix mismatched backticks in CONTRIBUTING.md (#6951)

It's rendered correctly at least on GitHub, but syntax highlighting fails after that point.

CONTRIBUTING.md

@@ -269,7 +269,7 @@ To list the available fuzzing harnesses you can run;
 $ cd tokio
 $ cargo fuzz list
 fuzz_linked_list
-````
+```
 
 Running a fuzz test is as simple as;