Add missing word 'to' [ci skip]

[henziger]

While the original sentence is easy to understand, adding the 'to' is
imho a bit more correct.

Sources:

• https://ell.stackexchange.com/q/16964
• https://www.usingenglish.com/forum/threads/127099-It-is-referred-as-or-It-is-referred-to-as

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @eileencodes (or someone else) soon.

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[kaspth]

Thanks!