Correcting a mistake in my journal article after it's been published
The official way of dealing with this (minor errors which don't substantially impact the conclusions of the paper) is with the errata/corrections section in the journal you published in. So you did the correct thing in contacting the editor about issuing a correction/erratum in a future issue of the journal.

Your problem at this point is the fact that the journal doesn't have a formal errata policy. There's two ways to proceed: 

The first is to attempt to convince the journal to establish an errata policy, for which you'll be the first example. This is ideal, but the editors might not be willing to do it, as it may involve substantially reworking how their journal is structured. Keep in mind that even if they accept the erratum, they might not actually change the published version of the article (even on their website), instead just adding the correction as a separate "erratum" notice/page.

The second is to simply post the erratum on your professional website, preferably on an official(ish) "errata" page. If the journal doesn't have an official errata policy, then this is probably the best option. It's used extensively by people who publish books (including textbooks). Having it on your own website may limit the visibility of the correction, but it will indicate to people who care that you're being proactive about correcting your mistakes to the best of your ability.