[range.cartesian.overview] Use `auto` instead of _`decay-copy`_

See https://github.com/cplusplus/draft/commit/ac52745c5b0c52090ba2254c214ea1bb54b43b19.
```suggestion
\tcode{auto(views::empty<tuple<>>)}
```

_Originally posted by @JohelEGP in https://github.com/cplusplus/draft/pull/5647#discussion_r929067952_