CONTRIBUTING guide: fix broken links and update "running tests for specific examples" section

[olliefr]

• Fix two broken links to section headers.
• Fix and expand the examples in the Running tests for specific examples section.

I recalled that I initially had some difficulties running the commands from that section. Now that I got the gist of it, I would like to propose a few changes. I fixed the examples that despite their description returned ambiguous results and added a few more examples of targeting specific tests.

Happy to provide more details, if need be 🙂

Checklist

I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass

[ludoo]

Thanks! You're on a roll :)