document wait seconds in build.js example

While solving an issue in my own project by increasing `waitSeconds`, I found that the parameter is missing the build.js example. So, the pull request adds the parameter to the example.

I think it would also be could to mention the parameter at http://requirejs.org/docs/errors.html#timeout because when running into the timeout, the optimizer mentions this link.

Note also that the I never had the issue when using requirejs 2.0.6. But when I upgraded to 2.1.4, I directly ran into it.