add "it" to make sentence more clear (pytorch#1689)

Co-authored-by: Holly Sweeney <[email protected]>

Author: yoosful

advanced_source/cpp_frontend.rst

@@ -1015,7 +1015,7 @@ probabilities.
 	is called *label smoothing*.
 
 Before evaluating the discriminator, we zero out the gradients of its
-parameters. After computing the loss, we back-propagate through the network by
+parameters. After computing the loss, we back-propagate it through the network by
 calling ``d_loss.backward()`` to compute new gradients. We repeat this spiel for
 the fake images. Instead of using images from the dataset, we let the generator
 create fake images for this by feeding it a batch of random noise. We then