Update working_with_lists.md

Author: sophiajt

book/working_with_lists.md

@@ -43,7 +43,7 @@ let colors = []
 = $colors | empty? # true
 ```
 
-The `in` and `not in` operators are used to test whether a value is in a list. For example:
+The `in` and `not-in` operators are used to test whether a value is in a list. For example:
 
 ```bash
 let colors = [red green blue]