Drop na.rm=TRUE when data is already purged of NAs (#811)

In transform.Rmd (Chapter 5, Data Transformation), under Counts (5.6.3),
we removed na.rm = TRUE when computing mean because all NAs for
arr_delay were already removed when creating not_cancelled dataset.

transform.Rmd

@@ -592,7 +592,7 @@ The story is actually a little more nuanced. We can get more insight if we draw
 delays <- not_cancelled %>% 
   group_by(tailnum) %>% 
   summarise(
-    delay = mean(arr_delay, na.rm = TRUE),
+    delay = mean(arr_delay),
     n = n()
   )