docs(slide.R): correct error message regarding data.frame rownames

The error message says that slide computations can't be rownamed data frames.
This is both bad wording (having rownames != having non-automatic rownames) and
we're not actually enforcing any restriction of the sort anyway. We drop/ignore
the rownames though (via dplyr/tibble). In `epix_slide`, the allowance of
non-automatic rownames was deliberate; in `epi_slide`, it also seems likely the
way to go; one might convert to `data.frame` and have some filter-like operation
return non-automatic rownames.

Author: brookslogan

R/slide.R

@@ -410,7 +410,7 @@ epi_slide_one_group <- function(
   # Returned values must be data.frame or vector.
   if ("other" %in% return_types) {
     cli_abort(
-      "epi_slide: slide computations must always return either data frames without rownames
+      "epi_slide: slide computations must always return either data frames
       or unnamed vectors (as determined by the vctrs package).",
       class = "epiprocess__invalid_slide_comp_value"
     )