Fix outdated msgpack-rpc doc.

Needed after neovim/python-client#58.

runtime/doc/msgpack_rpc.txt

@@ -113,9 +113,8 @@ string 'hello world!' on the current nvim instance:
 A better way is to use the python REPL with the `neovim` package, where API
 functions can be called interactively:
 >
-    >>> from neovim import socket_session, Nvim
-    >>> session = socket_session('[address]')
-    >>> nvim = Nvim.from_session(session)
+    >>> from neovim import attach
+    >>> nvim = attach('socket', path='[address]')
     >>> nvim.command('echo "hello world!"')
 <
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