typo in iteration.Rmd (#730)

iteration.Rmd

@@ -953,10 +953,10 @@ x %>% accumulate(`+`)
 1.  Implement your own version of `every()` using a for loop. Compare it with
     `purrr::every()`. What does purrr's version do that your version doesn't?
 
-1.  Create an enhanced `col_sum()` that applies a summary function to every
+1.  Create an enhanced `col_summary()` that applies a summary function to every
     numeric column in a data frame.
 
-1.  A possible base R equivalent of `col_sum()` is:
+1.  A possible base R equivalent of `col_summary()` is:
 
     ```{r}
     col_sum3 <- function(df, f) {