doc: update README.md to fix active/maint times

Update REAMDE.md to be consistent with the
active/maintenance times (12/18) outlined in
https://github.com/nodejs/Release#release-plan. This was missed
when the times were swapped in the Node.js 12 timeframe.

PR-URL: https://github.com/nodejs/node/pull/30707


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>