docs: Fix markdown commit message block in CONTRIBUTING

The code block for the commit message contains blank characters that are
breaking the markdown rendering engine, making the message block to
stop right after the first paragraph, and getting rid of the last line.

Simply cleaning these blank characters make the code block to render
properly. Note that I've also specified the type of the block, which is
plain text.

CONTRIBUTING.md

@@ -11,15 +11,17 @@ Please note we have a code of conduct, please follow it in all your interactions
    the pull request.
 2. If you're adding new API, it must be properly documented.
 3. The commit message is formatted as follows:
-   ```
+
+   ```plain
    component: <summary>
-   ‌
+
    A paragraph explaining the problem and its context.
-‌
+
    Another one explaining how you solved that.
-‌
+
    <link to the issue>
    ```
+
 4. You may merge the pull request in once you have the sign-off of the maintainers, or if you
    do not have permission to do that, you may request the second reviewer to merge it for you.