[Update DnD Character] Add Missing Function to Stub (#3735)

Per [this discussion](https://forum.exercism.org/t/d-d-character-test-bug/11237/3) on the forum.  Decided to amend the stub, as it was the easiest an least confusing solution.

Author: BethanyG

exercises/practice/dnd-character/dnd_character.py

@@ -1,3 +1,6 @@
 class Character:
     def __init__(self):
         pass
+
+def modifier(value):
+    pass