Chapter 13 (numbers): minor updates (hadley#1627)

* numbers.qmd: improve variable name for IQR

* numbers.qmd: textual fix

numbers.qmd

@@ -643,11 +643,11 @@ But the code below reveals a data oddity for airport [EGE](https://en.wikipedia.
 flights |> 
   group_by(origin, dest) |> 
   summarize(
-    distance_sd = IQR(distance), 
+    distance_iqr = IQR(distance), 
     n = n(),
     .groups = "drop"
   ) |> 
-  filter(distance_sd > 0)
+  filter(distance_iqr > 0)
 ```
 
 ### Distributions
@@ -717,7 +717,7 @@ Finally, don't forget what you learned in @sec-sample-size: whenever creating nu
 
 There's one final type of summary that's useful for numeric vectors, but also works with every other type of value: extracting a value at a specific position: `first(x)`, `last(x)`, and `nth(x, n)`.
 
-For example, we can find the first and last departure for each day:
+For example, we can find the first, fifth and last departure for each day:
 
 ```{r}
 flights |>