Fix bad anchor link in CONTRIBUTING.md

Hey look, I'm contributing to CONTRIBUTING.md!

This anchor must have been overlooked while updating the docs in a previous commit. Expected behavior: scroll the document to a linked section. Experienced behavior: nada.

This proposed file change renames the anchor text from `coding conventions` to `coding guidelines` and changes the anchor link from `#code-conventions` to `#code-guidelines`.
Merge pull request #12133 from leoj3n/patch-1

Fix bad anchor link in CONTRIBUTING.md