Is it possible to make minor (making a space between two words) proof corrections after already submitting a proof reading to a journal?
Email the production editor as soon as possible that you've found this extra correction. If it's still not inconvenient for them, they'll probably fix it. However, because it's such a minor error, you should be ready for them to say "it's too late" as well. This probably won't lead to an errata or corrigendum - it's just too minor.

If your previous reading had contained lots and lots of corrections, it's possible they're already planning to send you a set of second proofs.