fix code error and rename file to align with tutorial

Author: Lonercode

python-docstrings/doctest_style.py python-docstrings/magic_spells.pypython-docstrings/doctest_style.py renamed to python-docstrings/magic_spells.py

@@ -9,4 +9,4 @@ def undo_spell(spell):
     >>> undo_spell("Lumos")
     "somuL"
     """
-    return spell[::1]
+    return spell[::-1]