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: #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>

doc/node.1

@@ -442,7 +442,7 @@ Print stack traces for process warnings (including deprecations).
 .It Fl -track-heap-objects
 Track heap object allocations for heap snapshots.
 .
-.It Fl --unhandled-rejections=mode
+.It Fl -unhandled-rejections=mode
 Define the behavior for unhandled rejections. Can be one of `strict` (raise an error), `warn` (enforce warnings) or `none` (silence warnings).
 .
 .It Fl -use-bundled-ca , Fl -use-openssl-ca