Docs: Updated a memoryview example with a more meaningful numpy array initialization.

docs/examples/userguide/memoryviews/slicing.pyx

 import numpy as np
 
-exporting_object = np.empty((15, 10, 20), dtype=np.intc)
+exporting_object = np.arange(0, 15 * 10 * 20, dtype=np.intc).reshape((15, 10, 20))
 
 cdef int[:, :, :] my_view = exporting_object