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. Vim showed the special character with the string <300c>. I don't remember seeing it before to know where it came from.

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