add sobel filter image

Author: cezannec

convolutional-neural-networks/conv-visualization/custom_filters.ipynb

@@ -63,7 +63,7 @@
     "\n",
     "The Sobel filter is very commonly used in edge detection and in finding patterns in intensity in an image. Applying a Sobel filter to an image is a way of **taking (an approximation) of the derivative of the image** in the x or y direction, separately. The operators look as follows.\n",
     "\n",
-    "<img src=\"images/sobel_ops.png\" width=200 height=200>\n",
+    "<img src=\"notebook_ims/sobel_ops.png\" width=200 height=200>\n",
     "\n",
     "**It's up to you to create a Sobel x operator and apply it to the given image.**\n",
     "\n",