Update .CONTRIBUTING.md

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

Author: brunopgalvao

.CONTRIBUTING.md

@@ -61,7 +61,7 @@ Before making contributions, you need to set up the proper directory structure f
    > **polkadot-mkdocs**: Contains the "engine" (MkDocs configuration, theme, styling)
 
    ```bash
-   git clone https://github.com/papermoonio/polkadot-mkdocs
+   git clone https://github.com/papermoonio/polkadot-mkdocs.git
    cd polkadot-mkdocs
    ```