[skip changelog][skip ci] Fix contributing guidelines link on tests readme

[per1234]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests before creating one)
• The PR follows our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce?

Docs update.

• What is the current behavior?

Contributing guidelines link on tests readme is broken.

• What is the new behavior?

Contributing guidelines link on tests readme is not broken.

• Does this PR introduce a breaking change?

No

• Other information:

Linked to the Markdown file rather than the documentation site page so the link will always point to the matching docs.