Add missing word 'to' [ci skip]
Add missing word 'to' [ci skip]

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

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