Standardize map() when .f is an index rather than a function

[rossellhayes]

list <- list(
  c(1, 2),
  c("a", "b")
)

call <- quote(purrr::map(list, 2))

gradethis:::call_standardise_formals(call)
#> purrr::map(.x = list, .f = 2, .progress = FALSE)

^{Created on 2023-06-13 with reprex v2.0.2}

Closes #358.