doc: fix dgram.md typos

socket.getRecvBufferSize() and socket.getSendBufferSize()
do not take any parameter

PR-URL: https://github.com/nodejs/node/pull/16183


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>