Fix typo. Fixes #567

iteration.Rmd

@@ -610,7 +610,7 @@ If you're familiar with the apply family of functions in base R, you might have
     additional argument that defines the type. The only problem with 
     `vapply()` is that it's a lot of typing: 
     `vapply(df, is.numeric, logical(1))` is equivalent to
-    `map_lgl(df, is.numeric)`. One of advantage of `vapply()` over purrr's map
+    `map_lgl(df, is.numeric)`. One advantage of `vapply()` over purrr's map
     functions is that it can also produce matrices --- the map functions only 
     ever produce vectors.