Update callAsFunction API doc link in model_training_walkthrough.ipynb (

tensorflow#353)

Update copyright years to `2018 - 2020`.

docs/site/tutorials/model_training_walkthrough.ipynb

@@ -7,7 +7,7 @@
     "id": "QyCcF45zBQ3E"
    },
    "source": [
-    "##### Copyright 2018 The TensorFlow Authors. [Licensed under the Apache License, Version 2.0](#scrollTo=y_UVSRtBBsJk)."
+    "##### Copyright 2018 - 2020 The TensorFlow Authors. [Licensed under the Apache License, Version 2.0](#scrollTo=y_UVSRtBBsJk)."
    ]
   },
   {
@@ -482,7 +482,7 @@
     "\n",
     "The [Swift for TensorFlow Deep Learning Library](https://github.com/tensorflow/swift-apis) defines primitive layers and conventions for wiring them together, which makes it easy to build models and experiment.\n",
     "\n",
-    "A model is a `struct` that conforms to [`Layer`](https://www.tensorflow.org/swift/api_docs/Protocols/Layer), which means that it defines a [`callAsFunction(_:)`](https://www.tensorflow.org/swift/api_docs/Protocols/Layer#call_:) method that maps input `Tensor`s to output `Tensor`s. The `callAsFunction(_:)` method often simply sequences the input through sublayers. Let's define an `IrisModel` that sequences the input through three [`Dense`](https://www.tensorflow.org/swift/api_docs/Structs/Dense) sublayers."
+    "A model is a `struct` that conforms to [`Layer`](https://www.tensorflow.org/swift/api_docs/Protocols/Layer), which means that it defines a [`callAsFunction(_:)`](https://www.tensorflow.org/swift/api_docs/Protocols/Layer#callasfunction_:) method that maps input `Tensor`s to output `Tensor`s. The `callAsFunction(_:)` method often simply sequences the input through sublayers. Let's define an `IrisModel` that sequences the input through three [`Dense`](https://www.tensorflow.org/swift/api_docs/Structs/Dense) sublayers."
    ]
   },
   {