Fix usage of correct ScriptModule (pytorch#740)

Co-authored-by: holly1238 <[email protected]>

advanced_source/cpp_export.rst

@@ -115,7 +115,7 @@ If you need to exclude some methods in your ``nn.Module``
 because they use Python features that TorchScript doesn't support yet,
 you could annotate those with ``@torch.jit.ignore``
 
-``my_module`` is an instance of
+``sm`` is an instance of
 ``ScriptModule`` that is ready for serialization.
 
 Step 2: Serializing Your Script Module to a File
@@ -132,7 +132,7 @@ on the module and pass it a filename::
   traced_script_module.save("traced_resnet_model.pt")
 
 This will produce a ``traced_resnet_model.pt`` file in your working directory.
-If you also would like to serialize ``my_module``, call ``my_module.save("my_module_model.pt")``
+If you also would like to serialize ``sm``, call ``sm.save("my_module_model.pt")``
 We have now officially left the realm of Python and are ready to cross over to the sphere
 of C++.