phone-number: fix ImportError in slug file (#2152)

Template was created and renamed the class Phone to PhoneNumber... the example.py was already updated, but the slug file was overlooked.

Fixes #2151

exercises/phone-number/phone_number.py

@@ -1,3 +1,3 @@
-class Phone:
+class PhoneNumber:
-    def __init__(self, phone_number):
+    def __init__(self, number):
         pass