doc: fix typo in dgram doc

There is a typographical error in the dgram documentation. Reword to
eliminate the error and increase clarity.

PR-URL: https://github.com/nodejs/node/pull/11186


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>