doc: fix typo about cluster doc, (eg. -> e.g.)

Fixes: https://github.com/nodejs/code-and-learn/issues/58
PR-URL: https://github.com/nodejs/node/pull/9568


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Shigeki Ohtsu <ohtsu@ohtsu.org>