The help on editing lists some circumstances in which it's okay to edit posts (i.e. questions and answers). These include:
- To fix grammar and spelling mistakes
- To clarify the meaning of the post (without changing that meaning)
The first suggests that typos should probably be fixed, and the second clarifies that we must be careful to preserve the intent.
In this specific case, the intent was pretty obvious, and I think it's an error of the same kind as a 1-letter typo that results in a completely different word which results in a grammatically possible but plainly unintended meaning - the error is minor (the intent being correct aside from a typo), but a naive reading would be grossly misleading. Generally it's better if the person posting the answer fixes it, but if that may not happen for a long time (or at all), it would seem to be better to edit in spite of the limitation.
While one-letter corrections are to be avoided, where a naive reader would be likely to be misled, and the OP isn't likely to change it soon, I think an edit is okay, and indeed, in keeping with the sense I get from the guidelines in the help ... but I think it's incumbent on us to carefully label the reason for the change when making the edit, and generally, to highlight the change in comments.
I think this is broadly consistent with the help.