Fix typo

Author: mine-cetinkaya-rundel

factors.qmd

@@ -139,7 +139,7 @@ gss_cat |>
 When working with factors, the two most common operations are changing the order of the levels, and changing the values of the levels.
 Those operations are described in the sections below.
 
-### Exercise
+### Exercises
 
 1.  Explore the distribution of `rincome` (reported income).
     What makes the default bar chart hard to understand?