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

### Acknowledgement

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

### Comment

The `@param` tag [here](https://github.com/microsoft/TypeScript/blob/b465fdbfe175304d9b977da137b2c178ae1091d3/src/lib/es2022.array.d.ts#L4) 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](https://github.com/microsoft/TypeScript/blob/main/src/lib/es2022.string.d.ts#L4). I'd make a PR but there was an explicit recommendation not to do that for typos.

Start in src/lib/es2022.array.d.ts at the @param documentation for Array.prototype.at, then compare it with src/lib/es2022.string.d.ts. Update the array documentation to use the appropriate term for an array value, and verify the declaration-file documentation remains consistent.

Written by the indexing model from the issue text.