use stat() instead of .. in "3.7 Statistical transformations" (#751)

https://r4ds.had.co.nz/data-visualisation.html#statistical-transformations

visualize.Rmd

@@ -522,7 +522,7 @@ This works because every geom has a default stat; and every stat has a default g
     
     ```{r}
     ggplot(data = diamonds) + 
-      geom_bar(mapping = aes(x = cut, y = ..prop.., group = 1))
+      geom_bar(mapping = aes(x = cut, y = stat(prop), group = 1))
     ```
 
     To find the variables computed by the stat, look for the help section