Fix typo in 19.4 (#1585)

missing-values.qmd

@@ -215,7 +215,7 @@ And we want to count the number of smokers with `dplyr::count()`:
 health |> count(smoker)
 ```
 
-This dataset only contains non-smokers, but we know that smokers exist; the group of non-smoker is empty.
+This dataset only contains non-smokers, but we know that smokers exist; the group of non-smokers is empty.
 We can request `count()` to keep all the groups, even those not seen in the data by using `.drop = FALSE`:
 
 ```{r}