fix whitespace in last example

Author: jcjohnson

README.md

@@ -690,4 +690,5 @@ for t in range(500):
   # Zero gradients, perform a backward pass, and update the weights.
   optimizer.zero_grad()
   loss.backward()
-  optimizer.step()```
+  optimizer.step()
+```

nn/dynamic_net.py

@@ -67,4 +67,4 @@ def forward(self, x):
   # Zero gradients, perform a backward pass, and update the weights.
   optimizer.zero_grad()
   loss.backward()
-  optimizer.step()
+  optimizer.step()