Fix error in example of an RPC server

Sample code in the documentation for rpc.server has an error which
prevents the code from being run. Replacing the `self` with `None`
results in code that runs. Whether it does anything useful is
an open question.

Change-Id: I81391fa3efdc65d54ca3aa155d63584f2b6f98c2
Merge "Fix error in example of an RPC server"