GH-46717: [R][Docs] Add missing "internal" keywords for internal function (#46722)

### Rationale for this change

pkgdown generation was failing due to a function not being included in the list of functions to document

### What changes are included in this PR?

Update roxygen header to not generate that function or trigger the pkgdown check

### Are these changes tested?

Nah, but I'll trigger CI to check

### Are there any user-facing changes?

No
* GitHub Issue: #46717

Authored-by: Nic Crane <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: thisisnic

r/R/dplyr-funcs-agg.R

@@ -204,6 +204,7 @@ find_arrow_mask <- function() {
 #'   group_by(cyl) |>
 #'   summarize(x = one(disp))
 #' }
+#' @keywords internal
 one <- function(...) {
   set_agg(
     fun = "one",