[doc] Fix JIT code highlighting (pytorch#37338)

Summary:
Fix pytorch#36216

| Before | After |
| --- | --- |
| ![image](https://user-images.githubusercontent.com/6421097/80353700-55abec80-883b-11ea-9ae2-72f37ba23c16.png)| ![image](https://user-images.githubusercontent.com/6421097/80353403-ef26ce80-883a-11ea-885b-2a2963f79d20.png) |
Pull Request resolved: pytorch#37338

Differential Revision: D21262421

Pulled By: mrshenli

fbshipit-source-id: 4fb62cce9543e6a4852828f58a279c36565f8c44

docs/source/jit.rst

@@ -214,6 +214,8 @@ Disable JIT for Debugging
     you can use tools like ``pdb`` to debug the model code.
 
     Given an example 
+
+    .. testcode::
 
         @torch.jit.script
         def scripted_fn(x : torch.Tensor):