Fix typo chapter6-exercise-class-selectors-tests.md

src/chapter6-exercise-class-selectors-tests.md

@@ -23,7 +23,7 @@ If you’ve implemented them correctly, you should see code blocks in this book
     ...             val = pairs[key]
     ...             print(f"  {key}: {val}")
 
-To do this you will need to add a new class called `ClasSelector` to perform
+To do this you will need to add a new class called `ClassSelector` to perform
   the matching.
 
     >>> browser.ClassSelector("b").matches(