Update 'README.md'

README.md

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 ### Building a basic `PhoneBook` class
 
-![PhoneBookDesign]("PhoneBookUML.png")
+![PhoneBookDesign](./PhoneBookUML.png)
 
 * Create a `PhoneBook` class that holds names and phone numbers.
 * You can use an [associative data type](https://en.wikipedia.org/wiki/Associative_array) (one which stores items as keys paired with values).