fix link to CONTRIBUTING.md

[Konfekt]

See discussion at #5216

• I solemnly swear that this is all original content of which I am the original author
• Pull request title is prepended with [language/lang-code] (example [python/fr] or [java/en])
• Pull request touches only one file (or a set of logically related files with similar changes made)
• Content changes are aimed at intermediate to experienced programmers (this is a poor format for explaining fundamental programming concepts)
• If you've changed any part of the YAML Frontmatter, make sure it is formatted according to CONTRIBUTING.md. No, that's the point of this PR
  • Yes, I have double-checked quotes and field names! What are field names?

[Konfekt]

(Autitisc!?) contributors might feel compelled to at least read the contributing guidelines before honestly confirming the fourth point which leaves them in a limbo if the link is broken; replace the moving master branch commit pointer by a fixed one ensuring it will keep working

[Konfekt]

Yes, I have double-checked quotes and field names!

For similar reasons, as someone only peripherally involved with the development of this content platform, additional context when referring to "field names"; meant are perhaps those of the YAML front matter? How about adding this information as well?

[verhovsky]

Yes, I have double-checked quotes and field names!

I replaced that note with a linter in #5218