Fix "off by two" error when checking for args

Created by: mrnugget

I don't know how I previously came up with 4, but as it turns out if you run this

src campaigns add-changesets -campaign='Q2FtcGFpZ246MQ==' -repo-name=github.com/sourcegraph/sourcegraph 33 44 55 66

The IDs 33 and 44 are ignored.

This fixes the issue.