doc: fix --enable-source-maps flag in v12.12.0 changelog

Corrects "--source-map-support" to "--enable-source-maps".

Fixes: https://github.com/nodejs/node/issues/29959
PR-URL: https://github.com/nodejs/node/pull/29960


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>