pkgdown site links `as_factor()` to `forcats::as_factor()` instead of `haven::as_factor()`

[dtburk]

The value-labels vignette refers to as_factor(), and pkgdown attempts to automatically link this to the appropriate function documentation, but in this case, it links to forcats::as_factor() instead of haven::as_factor(). If the vignette was just referring to a function from an external package, we could just use haven::as_factor() explicitly. However, ipumsr re-exports haven::as_factor() so that ipumsr users don't have to load haven to use it, so it wouldn't be ideal if we had to use haven::as_factor() in the vignette just to get the pkgdown link to work properly.

One partial solution would be to replace references to as_factor() with

[`as_factor()`](https://haven.tidyverse.org/reference/as_factor.html)

in the text of the vignette, but then those links would look different from the links auto-generated by pkgdown, and we would have to manually update the url if haven ever moved its documentation site. Moreover, that approach wouldn't work for code references to as_factor().

It's possible that we should create an issue on pkgdown or downlit requesting a new feature that allows pkgdown users to manually specify which package a function is from for function names that appear in multiple packages, or alternatively, an update that checks for function name matches in re-exported functions before looking more widely.