Fix grammar

Change 'You want to create a class methods and instance methods'
to 'You want to create class and instance methods'

Author: Zach Boatrite

chapters/classes_and_objects/class-methods-and-instance-methods.md

@@ -5,7 +5,7 @@ chapter: Classes and Objects
 ---
 ## Problem
 
-You want to create a class methods and instance methods.
+You want to create class and instance methods.
 
 ## Solution