Adds missing of to transform.Rmd

transform.Rmd

@@ -277,7 +277,7 @@ There are a number of helper functions you can use within `select()`:
 
 See `?select` for more details.
 
-`select()` can be used to rename variables, but it's rarely useful because it drops all the variables not explicitly mentioned. Instead, use `rename()`, which is a variant of `select()` that keeps all the variables that aren't explicitly mentioned:
+`select()` can be used to rename variables, but it's rarely useful because it drops all of the variables not explicitly mentioned. Instead, use `rename()`, which is a variant of `select()` that keeps all the variables that aren't explicitly mentioned:
 
 ```{r}
 rename(flights, tail_num = tailnum)