add a reference for Laplacian pyramid

tensorflow/examples/tutorials/deepdream/deepdream.ipynb

@@ -620,7 +620,7 @@
     "<a id=\"laplacian\"></a>\n",
     "## Laplacian Pyramid Gradient Normalization\n",
     "\n",
-    "This looks better, but the resulting images mostly contain high frequencies. Can we improve it? One way is to add a smoothness prior into the optimization objective. This will effectively blur the image a little every iteration, suppressing the higher frequencies, so that the lower frequencies can catch up. This will require more iterations to produce a nice image. Why don't we just boost lower frequencies of the gradient instead? One way to achieve this is through the Laplacian pyramid decomposition. We call the resulting technique _Laplacian Pyramid Gradient Normailzation_."
+    "This looks better, but the resulting images mostly contain high frequencies. Can we improve it? One way is to add a smoothness prior into the optimization objective. This will effectively blur the image a little every iteration, suppressing the higher frequencies, so that the lower frequencies can catch up. This will require more iterations to produce a nice image. Why don't we just boost lower frequencies of the gradient instead? One way to achieve this is through the [Laplacian pyramid](https://en.wikipedia.org/wiki/Pyramid_(image_processing)#Laplacian_pyramid) decomposition. We call the resulting technique _Laplacian Pyramid Gradient Normailzation_."
    ]
   },
   {