doc: switch the order of Writable and Readable

This change places the links to Writable stream and Readble stream
in the order these sections appear when scrolling down the screen.

PR-URL: https://github.com/nodejs/node/pull/21333


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>