Swapped buffer and target array in example 021

Thank you so much for your contribution related to pyOpenCL. Helps a lot. I tried your example
https://github.com/benshope/PyOpenCL-Tutorial/blob/master/021_array_sum.py

but the variables seem to have been swapped on line 39? 

`cl.enqueue_copy(queue, cl_arrays[x], np_arrays[x])` 

should probably read: 

`cl.enqueue_copy(queue, np_arrays[x], cl_arrays[x])`

Best regards
Olle