Update 'README.md'

README.md

 
 	* `reverseLookup(String phoneNumber)`
 		* returns a name for the respective input `phoneNumber`
-	* `display()`
-		* print a human-readable list of all entries (names and phone numbers) in alphabetical order.
+	* `toString()`
+		* return a human-readable list of all entries (names and phone numbers) in alphabetical order.
 
 	   * Sample Script
 	
 		   PhoneBook phoneBook = new PhoneBook();
 		   phoneBook.add("Zebra", "111-222-333");
 		   phoneBook.add("Dog", "222-444-4444");
-		   phoneBook.display(); 
+		   
+           String entries = phoneBook.toString(); 
+           System.out.println(entries);
 		   ```
 	
 	   * Sample Output