Minor clarification. (#1012)

Delayed and cancelled flights are not the same thing.

Co-authored-by: Christian Hohenfeld <git@hohenfeld.is>

numbers.Rmd

@@ -205,7 +205,7 @@ flights |>
   )
 ```
 
-We can combine that with the `mean(is.na(x))` trick from Section \@ref(logical-summaries) to see how the proportion of delayed flights varies over the course of the day.
+We can combine that with the `mean(is.na(x))` trick from Section \@ref(logical-summaries) to see how the proportion of cancelled flights varies over the course of the day.
 The results are shown in Figure \@ref(fig:prop-cancelled).
 
 ```{r prop-cancelled}