doc: link to readable and writeable stream section

Modify the `[Writable]` and `[Readable]` links so they point
directly to the right sections in the stream.html doc

PR-URL: https://github.com/nodejs/node/pull/11517


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>