doc: use "Long Term Support" in collaborator guide

Use "Long Term Support" rather than "Long-term support".

Refs: https://github.com/nodejs/node/pull/38839#issuecomment-850853132

PR-URL: https://github.com/nodejs/node/pull/38841


Reviewed-By: Derek Lewis <DerekNonGeneric@inf.is>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>