Update documentation to include proper class name formatting and pron…

[cjilbert504]

…oun problem fix

Motivation / Background

This Pull Request has been created to clarify the full class name to inherit from when creating additional system test classes and because of a pronoun problem.

Detail

This Pull Request changes the documentation to ensure accuracy by including the full class name with formatting. Previously, the class name ActionDispatch::SystemTestCase was missing proper formatting and the module name, resulting in potential confusion. Additionally, the commit addresses a pronoun fix by replacing "use in your test suite" with "use it in your test suite."

Additional information

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[jonathanhefner]

Thank you, @cjilbert504! 📜

Backported to 7-0-stable.

[cjilbert504]

Thank you, @jonathanhefner!!! 🖋️