Fix incorrect doc in conv2d_nhwc_python (#1677)

Author: vinx13

topi/python/topi/testing/conv2d_nhwc_python.py

@@ -13,7 +13,7 @@ def conv2d_nhwc_python(a_np, w_np, stride, padding):
         4-D with shape [batch, in_height, in_width, in_channel]
 
     w_np : numpy.ndarray
-        4-D with shape [num_filter, filter_height, filter_width, in_channel]
+        4-D with shape [filter_height, filter_width, in_channel, num_filter]
 
     stride : int or a list/tuple of two ints
         Stride size, or [stride_height, stride_width]