Modify linear regression loss function (#373)

* Modify tf2 linear regression loss function

* neural_network.ipynp syntax error has been corrected

tensorflow_v2/notebooks/3_NeuralNetworks/neural_network.ipynb

@@ -116,7 +116,7 @@
     "    # Set forward pass.\n",
     "    def call(self, x, is_training=False):\n",
     "        x = self.fc1(x)\n",
-    "        x = self.fc2(x)\n"
+    "        x = self.fc2(x)\n",
     "        x = self.out(x)\n",
     "        if not is_training:\n",
     "            # tf cross entropy expect logits without softmax, so only\n",