[docs] Fix typos, heading case, and outdated description in CONTRIBUT…

[tangclfs]

Description

Fix typos, heading case, and outdated description in CONTRIBUTING.md

Self-check

• This is my first time contributing, I've carefully read CONTRIBUTING.md
  and I've updated CONTRIBUTORS with my name.
• If this is a fix, user-facing change, a compiler change, or a new paper
  implementation, I have updated CHANGELOG_NEXT.md
• I confirm that this contribution did not involve GenerativeAI nor Large Language Models.