doc: make `writable.setDefaultEncoding()` return `this`

Let this function return `this` for parity with `readable.setEncoding()`.

PR-URL: https://github.com/nodejs/node/pull/5040
Fixes: https://github.com/nodejs/node/issues/5013



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