Review potentially copy pasted content in class_extension/relation_to_encapsulation

https://javabook.mccue.dev/class_extension/relation_to_encapsulation
u might want to review if there is copy pasta content on this page. if not, maybe reword some paragraph. i tripped over
>A reasonable extension to this class would be to print a message every time someone books a flight.1    

which is repeated literally

originally suggested in the TJ discord by zabuzard.