CONTRIBUTING.md: formatting guidelines
CONTRIBUTING.md: formatting guidelines

- Enable C syntax highlightin in the example
- Update tabs recommendation (keep existing styles, move to tab stop of
  4 spaces)

Thanks to @fniephaus and @nicolas-cellier-aka-nice for their feedback.

OpenSmalltalk/vm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm,OpenSmalltalk/vm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm,OpenSmalltalk/vm,OpenSmalltalk/vm,OpenSmalltalk/vm,OpenSmalltalk/vm,OpenSmalltalk/vm,OpenSmalltalk/vm