added format to encode real PIDs got from previous rpc call

I found if you try to encode a PID got from previous rpc call it does not work.
In peb_encode documentation I found that ~p format is for erlang pid but indeed it is for a peb bridge resource (the result of peb_connect) as you can see in the example in the peb_send_bypid documentation.

I created a new "~n" format to encode a real erlang pid. May be it should be "~p" and replace the current one for other letter ;)