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.

PR-URL: https://github.com/nodejs/node/pull/28431
Refs: https://github.com/nodejs/node/pull/28007
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

doc/api/stream.md

@@ -505,7 +505,7 @@ the status of the `highWaterMark`.
 
 ##### writable.writableFinished
 <!-- YAML
-added: v12.4.0
+added: REPLACEME
 -->
 
 * {boolean}