"octopi" is incorrect

The `-us` ↔ `-i` forms come from Latin.

The word `octopus` is of Greek origin. It's Greek plural form is `octopodes`, but in English the word is commonly pluralized according to English rules: `octopuses`.

Since so many people fall for this error, some dictionaries mark `octopi` as a possible pluralization. Nevertheless, we should not support this common mistake.

Both the logic and the readme need to be fixed.

Prooflinks:

* https://grammarist.com/usage/octopi-octopuses/
* https://en.wikipedia.org/wiki/Octopus