I think the software gives you a limited amount of time to edit. The general purpose of this is to preserve the continuity of discussion.

If it's something informational like this, you could just post a correction in the same thread, or if the correction would appear several pages after the wrong information or if it's a serious thing (like inadvertantly posting something that could be slanderous or offensive), you could PM the moderators with a link to the post to be corrected or use the "report post" button to report your own post with the correction.