Update the ReLU Link (tensorflow#498)

Dan Hurt found that the ReLU link was not pointing to the correct place.

docs/site/tutorials/model_training_walkthrough.ipynb

@@ -541,7 +541,7 @@
     "id": "fK0vrIRv_tcc"
    },
    "source": [
-    "The activation function determines the output shape of each node in the layer. These non-linearities are important—without them the model would be equivalent to a single layer. There are many available activations, but [ReLU](https://www.tensorflow.org/swift/api_docs/Functions#/s:10TensorFlow4reluyAA0A0VyxGAESFRzAA0aB6ScalarRzlF) is common for hidden layers.\n",
+    "The activation function determines the output shape of each node in the layer. These non-linearities are important—without them the model would be equivalent to a single layer. There are many available activations, but [ReLU](https://www.tensorflow.org/swift/api_docs/Functions#relu_:) is common for hidden layers.\n",
     "\n",
     "The ideal number of hidden layers and neurons depends on the problem and the dataset. Like many aspects of machine learning, picking the best shape of the neural network requires a mixture of knowledge and experimentation. As a rule of thumb, increasing the number of hidden layers and neurons typically creates a more powerful model, which requires more data to train effectively."
    ]