Fix docs: `jhu_csse_daily_subset` is an `epi_df`, not just a tibble

[brookslogan]

Roxygen docstrings for jhu_csse_daily_subset states under "Format:" that it's a tibble, but more specifically, it's an epi_df. We should describe it as the latter, not the former. Docstrings for other toy data sets included should also be double-checked. (Rd files should then be updated via devtools::document().)