Do you know that you can edit a post if you catch a typo within 15 minutes? Click on your post (as it appears in the message list) and that brings the post up. There are a bunch of links in blue right above the top edge of the message -- you can click the EDIT link -- IF you do this within 15 minutes of posting, and you can fix your typo that way.

However if you're posting through the mobile app, evidently you CAN'T edit your post.

If ever someone has a post that they really goofed up, they can ask a moderator to delete it, and then they can re-post what they really meant to say.