Update old function name (#1233)

The function `append_csv()` is nowhere to be found the rest of the book. I assume from the context that `append_csv` is an earlier name of what is now `append_file`

iteration.qmd

@@ -881,7 +881,7 @@ append_file <- function(path) {
 }
 ```
 
-Now we need to call `append_csv()` once for each element of `paths`.
+Now we need to call `append_file()` once for each element of `paths`.
 That's certainly possible with `map()`:
 
 ```{r}