`Array.prototype.at` docs use "code unit" instead of "item"

[grundb]

Acknowledgement

• I acknowledge that issues using this template may be closed without further explanation at the maintainer's discretion.

Comment

The @param tag here in the docs for Array.prototype.at uses the term "code unit" instead of the more appropriate term "item" or "element". It was presumably copied from the docs for String.prototype.at here. I'd make a PR but there was an explicit recommendation not to do that for typos.

[RyanCavanaugh]

Copyediting for correctness in lib.d.ts is fine