doc: use consistent naming in stream doc

Consistency of method naming

referred to as readable.push several other times in transform
documentation and also documented under readable, so makes sense
to just stick with readable.push

PR-URL: https://github.com/nodejs/node/pull/30506


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>