Fix typos in RPC examples (#40280)
Summary:
There has a missing '=' in rpc_sync call in RPC example.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40280
Differential Revision: D22137619
Pulled By: mrshenli
fbshipit-source-id: f4e4b85f68fd68d29834e199416176454b6bbcc2
