Fixed bug in tail recursion example.

A misplaced right-paren checked for the length of a boolean and then
conditioned an `if` statement on that length.

TailRecursion.py

@@ -10,7 +10,7 @@ def slightly_better_iterator(a_list):
 
 
 def poorly_defined_adder(a_list):
-    if len(a_list == 0):
+    if len(a_list) == 0:
         return 0
     else:
         return a_list[0] + poorly_defined_adder(a_list[1:])