Fixed incorrect description in delegation practice exercise

[alanloi]

Updated the exercise documentation on delegation to correctly refer to the Messenger interface instead of the SmartMessenger class.

Trying to implement the exercise as is will give the following compilation error and lead to user confusion:

Delegation is supported only for interfaces.
This type is final, so it cannot be extended.
This type has a constructor, so it must be initialized here.

[sarahhaggarty]

Great catch! Thanks for your contribution @alanloi !