Docs: Fix broken code syntax in autograd.rst (#69362)

Summary:
The backticks around `nn.Parameters` were not rendered correctly because the word was enclosed in an italics block.
Spotted the issue on https://pytorch.org/docs/stable/notes/autograd.html#locally-disable-grad-doc.

Pull Request resolved: #69362

Reviewed By: zou3519

Differential Revision: D32924093

Pulled By: albanD

fbshipit-source-id: 5a310ac3f3d13a5116f7aa911817b9452eee711d

docs/source/notes/autograd.rst

@@ -108,7 +108,7 @@ Setting ``requires_grad``
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 
 :attr:`requires_grad` is a flag, defaulting to false *unless wrapped
-in a ``nn.Parameter``*, that allows for fine-grained exclusion of
+in a* ``nn.Parameter``, that allows for fine-grained exclusion of
 subgraphs from gradient computation. It takes effect in both the
 forward and backward passes: