Fix pkgdown configuration for reference

[maelle]

We get

 Error : In '_pkgdown.yml', topic must be a known selector function
✖ Not 'osmplotr-package'

I think the easiest fix is to add

#' @keywords internal

for that topic then redocumenting.

[maelle]

@mpadge friendly reminder :smile_cat:

[mpadge]

sorry @maelle, i still can't run check_pkgdown() locally, so it's a bit more effort for me to run those checks. Anyway, i checked the above in a container, and all was good. Thanks

[maelle]

Thank you!!