Fix a typo: s/it/is

15_autoencoders.ipynb

@@ -520,7 +520,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "There are many ways to train one Autoencoder at a time. The first approach it to train each Autoencoder using a different graph, then we create the Stacked Autoencoder by simply initializing it with the weights and biases copied from these Autoencoders."
+    "There are many ways to train one Autoencoder at a time. The first approach is to train each Autoencoder using a different graph, then we create the Stacked Autoencoder by simply initializing it with the weights and biases copied from these Autoencoders."
    ]
   },
   {