Fix link on website

Author: mo8it

README.md

@@ -124,7 +124,7 @@ Continue practicing your Rust skills by building your own projects, contributing
 
 Do you want to create your own set of Rustlings exercises to focus on some specific topic?
 Or do you want to translate the original Rustlings exercises?
-Then follow the link to the guide about [third-party exercises](THIRD_PARTY_EXERCISES.md)!
+Then follow the link to the guide about [third-party exercises](https://github.com/rust-lang/rustlings/blob/main/THIRD_PARTY_EXERCISES.md)!
 
 ## Uninstalling Rustlings