doc: fix incorrect syntax in examples

The cluster docs had a period instead of a semicolon at the end of two
lines.

PR-URL: https://github.com/nodejs/node/pull/6463


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Jackson Tian <shvyo1987@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>