contributing.md: make MkDocs happier with a link

docs/contributing.md

@@ -41,7 +41,7 @@ test and document.
 The commit message should describe the changes in the commit;
 the PR description can even be empty, but feel free to include a personal
 message. We write commit messages in an affected component style and don't use
-[conventional commits](www.conventionalcommits.org/en/v1.0.0/). This means if
+[conventional commits](https://www.conventionalcommits.org/en/v1.0.0/). This means if
 you modified a command in the CLI, use its name as the topic, e.g
 `next/prev: <your-modification>` or `conflicts: <your-modification>`. We don't
 currently have a specific guidelines on what to write in the topic field, but