Fix-up moving average example.

Doc/library/collections.rst

@@ -455,10 +455,9 @@ added elements by appending to the right and popping to the left::
         # moving_average([40, 30, 50, 46, 39, 44]) --> 40.0 42.0 45.0 43.0
         # http://en.wikipedia.org/wiki/Moving_average
         it = iter(iterable)
-        d = deque(itertools.islice(it, n))
+        d = deque(itertools.islice(it, n-1))
+        d.appendleft(0)
         s = sum(d)
-        if len(d) == n:
-            yield s / n
         for elem in it:
             s += elem - d.popleft()
             d.append(elem)