doc: fix exec example in child_process

Previously, the example was checking for error by strict equality to
null. The error could be undefined though which would fail that check.

PR-URL: https://github.com/nodejs/node/pull/6660


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Jeremy Whitlock <jwhitlock@apache.org>