joins.qmd: fix party-birthday matching condition in text (fixes hadley#1610)

Author: florisvdh

joins.qmd

@@ -819,7 +819,7 @@ employees <- tibble(
 employees
 ```
 
-And for each employee we want to find the first party date that comes after (or on) their birthday.
+And for each employee we want to find the last party date that comes before (or on) their birthday.
 We can express that with a rolling join:
 
 ```{r}