docs: fix heading level for an option in class-methods-use-this rule

[chalkygames123]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[x] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

Just fix an incorrect heading level.

Is there anything you'd like reviewers to focus on?

No.

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Takuya Fukuju (747bf44)

[eslint-github-bot]

Hi @chalkygames123!, thanks for the Pull Request

The first commit message isn't properly formatted. We ask that you update the message to match this format, as we use it to generate changelogs and automate releases.

• The length of the commit message must be less than or equal to 72

To Fix: You can fix this problem by running git commit --amend, editing your commit message, and then running git push -f to update this pull request.

Read more about contributing to ESLint here

[snitin315]

LGTM, thank you.

[mdjermanovic]

LGTM, thanks for contributing!