doc: spell writable consistently

Docs have 108 instances of "writable" and only 1 "writeable" so
fix this one.

PR-URL: https://github.com/nodejs/node/pull/4954


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>