Remove math mode from lists page, no longer necessary

Author: rgwood

book/working_with_lists.md

@@ -43,10 +43,7 @@ let colors = []
 = $colors | empty? # true
 ```
 
-The `in` and `not in` operators are used to test whether a value is in a list.
-Operators can only be used in "math mode".
-One way to enter math mode is to begin an expression with `=`.
-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]