fix document for radio_button

[shingo-nakanishi]

• Add # Let's say that @user.receive_newsletter returns "no": for def radio_button(object_name, method, tag_value, options = {}).
• Fix # Let's say that @user.category returns "no": to # Let's say that @user.receive_newsletter returns "no":

[rails-bot]

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[vipulnsward]

Thanks!

@shingo-nakanishi please add a [ci skip] to document-only changes, so that we can avoid running builds on Travis CI for doc changes.

[shingo-nakanishi]

@vipulnsward Thank you! I will add a [ci skip] to document-only changes, next time.