An edit button carries the implicit promise that your changes will be read. But once the recipient has read what you wrote, they're unlikely to read it again.
Fine, then what if we also mark the changed message as unread again, then they will read it again, right? Except now I, as the reader, have to hunt for your change(s) to make sure I'm still correctly understanding what you said. Because the change may just be a typo, but even replacing just a single word can change the meaning of an entire message.
Typose are OK. Don't worry about it. And if you made a really big mistake, just send a second message.