Update stat_summary args for ggplot2 3.3.0 (#839)

visualize.Rmd

@@ -537,9 +537,9 @@ This works because every geom has a default stat; and every stat has a default g
     ggplot(data = diamonds) + 
       stat_summary(
         mapping = aes(x = cut, y = depth),
-        fun.ymin = min,
-        fun.ymax = max,
-        fun.y = median
+        fun.min = min,
+        fun.max = max,
+        fun = median
       )
     ```