Fix doc of `ActionDispatch::SystemTestCase` [ci skip]
Fix doc of `ActionDispatch::SystemTestCase` [ci skip]

* Fix broken format.
* Need to specify driver to the first argument of `driven_by`.
* `add_emulation` doesn't have `device` keyword. Ref: https://github.com/SeleniumHQ/selenium/blob/master/rb/lib/selenium/webdriver/chrome/options.rb#L142-L162

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