docs: update custom_classes.md

minor fix

Author: eltociear

reference/concepts/custom_classes.md

@@ -5,6 +5,6 @@ TODO: ADD MORE
 - the exercise relies on the `class` statement to create a custom class [allergies](../exercise-concepts/allergies.md)
 - use of `class` to create a custom data structure [clock](../exercise-concepts/clock.md)
 - the exercise objective is to define a `matrix` type. Tested methods are linked to a `matrix` class [matrix](../exercise-concepts/matrix.md)
-- a general comprehension of class concept and and how it works is required, `class` statement [binary-search-tree](../exercise-concepts/binary-search-tree.md)
+- a general comprehension of class concept and how it works is required, `class` statement [binary-search-tree](../exercise-concepts/binary-search-tree.md)
 - classes are defined with the `class <ClassName>:` syntax [phone-number](../exercise-concepts/phone-number.md)
 - the exercise objective is to define a `robot` type. Tested methods are linked to a `robot` class. [robot-simulator](../exercise-concepts/robot-simulator.md)