As it is a quote it should be preserved the way the original is - everything else is simply wrong from an ethical standpoint by saying "Some other person said this" when in fact you know they didn't. That the source is wrong is a different problem, but when you want to use this source or the author of the answer wants to use it, the quote should be preserved the way it is.
The usual thing to do in academic papers that need citations is to point out that something is different from the original or that something in the original is wrong and you realized it, but still want to quote it because it's still relevant.
You could for example add a [sic] after a typo to show that the typo was in the original text, that you have seen it and that you are aware that, according to current rules, this should be changed. For this specific example adding a [sic] and then explaining in the normal text after the quote why you added this little bit and what the difficulties with roll20 are would be perfect to show the reader what the problem is.
Another way would be to write something like [a]ttack and then mention why you changed the capitalization.
Or, to be very explicit, you could add something like:
Attack [this should be lowercase attack - see [this meta discussion](link/to/this/discussion) for more].
It might also be a good idea to add a comment, poiting out the problems and the correct capitalization that is used in books/ official sources. Maybe even poiting to a better source they can use for their claim. The OP can then decide whether to use the source you provided or the one they originally used.