doc: fix year in changelog

Most minor, 2018 should be 2019 for 10.15.3

PR-URL: https://github.com/nodejs/node/pull/26584


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>