doc: replace version with REPLACEME
Node 12.4.0 has already been released. Replace the version with REPLACEME so that the proper version gets inserted at release time.
Refs: https://github.com/nodejs/node/pull/28007
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines