Update .CONTRIBUTING.md

Co-authored-by: Erin Shaben <[email protected]>

Author: brunopgalvao

.CONTRIBUTING.md

@@ -71,7 +71,7 @@ Before making contributions, you need to set up the proper directory structure f
 
    ```bash
    # Fork polkadot-docs on GitHub first, then:
-   git clone https://github.com/your-username/polkadot-docs.git
+   git clone https://github.com/YOUR_USERNAME/polkadot-docs.git
    cd polkadot-docs
    git checkout -b your-feature-branch
    ```