Update pool.py

fix spell error.

Author: Svtter

creational/pool.py

@@ -17,7 +17,7 @@
 populated with strings.
 As we can see, the first string object put in "yam" is USED by the
 with statement. But because it is released back into the pool
-aftwerwards it is reused by the explicit call to sample_queue.get().
+afterwards it is reused by the explicit call to sample_queue.get().
 Same thing happens with "sam", when the ObjectPool created insided the
 function is deleted (by the GC) and the object is returned.