doc: consistent 'Returns:' part two

Follow up from 8eb19c48. Lower case `return` was not updated

PR-URL: https://github.com/nodejs/node/pull/10391
Ref: https://github.com/nodejs/node/pull/9554


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Julian Duque <julianduquej@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: James M Snell <jasnell@gmail.com>