doc: remove extraneous dash from flag prefix

When rendered by `man node`, all flags have a dash prepended to them.
Prior to this change, the man page would incorrectly read
`---unhandled-rejections=mode`

The three dashes is invalid syntax and rejected by the `node` command.

PR-URL: https://github.com/nodejs/node/pull/38532


Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>