luhn: fix lambda usage (see exercism#287)

Author: behrtam

luhn/example.py

@@ -3,8 +3,9 @@ def __init__(self, number):
         self.number = number
 
     def addends(self):
+        def luhn_transform(n):
+            return (2 * n - 9) if (n > 4) else (2 * n)
         old_digits = [int(d) for d in str(self.number)]
-        luhn_transform = lambda n: (2 * n - 9) if (n > 4) else (2 * n)
         return [(luhn_transform(n) if (i % 2 == 0) else n)
                 for i, n in enumerate(old_digits, start=len(old_digits) % 2)]