[Docs]: Need to update roxygen @param tag for the after arg of req_retry

[jimbrig]

I noticed the source code for req_retry is updated and now the after argument's callback function wants either NA or a number instead of the previous NULL.

To avoid confusion, the roxygen2 @param docs for the after argument should be updated to reflect this change:

Reference:

• Line #49 of req-retries.R
• Line #135 of req-retries.R

Error received:

Error in `httr2::req_perform()`:
! The `after` callback to `req_retry()` must return a single number or NA, not `NULL`.
Run `rlang::last_trace()` to see where the error occurred.

Current @param tag for the after argument of req_retry:

#' @param after A function that takes a single argument (the response) and
#'   returns either a number of seconds to wait or `NULL`, which indicates
#'   that a precise wait time is not available that the `backoff` strategy
#'   should be used instead..

should now be:

#' @param after A function that takes a single argument (the response) and
#'   returns either a number of seconds to wait or `NA`, which indicates
#'   that a precise wait time is not available that the `backoff` strategy
#'   should be used instead.

(changing NULL to NA)

[jimbrig]

made a quick PR: #598

not sure if you want to keep like this or maybe adjust the function to be numeric or NA or NULL?

regardless, appreciate the amazing job with this library! much smoother developer experience than the original httr library!