doc: use "Long Term Support" in README

"Long-Term Support" should have a capital "T" according to most style
guides if we're making it a proper noun as we are here. It's obviously
not super important in the grand scope of the project, but it makes
things appear more formal and correct.

PR-URL: https://github.com/nodejs/node/pull/38839


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>