Fix missing word in sentence

[dboulet]

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

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

What changes did you make? (Give an overview)
Very minor change to the documentation.

Is there anything you'd like reviewers to focus on?
No.

[jsf-clabot]

All committers have signed the CLA.

[kaicataldo]

Thanks for contributing to ESLint! Do you mind updating the commit message to the format we request in the commit-message check? Doing so helps us automate our release and change log creation process.

[dboulet]

Thanks, @kaicataldo. I’ve amended my commit message and all checks are now passing.

[platinumazure]

Looks good, thanks for contributing!