Editing, among many other areas of Stack Exchange site is a priviledge that can be earned by gaining reputation. As far as editing goes, before you earn 2000 reputation all edits you make will be added to a review queue for people to review and approve. Once you pass 2000 rep you can make all the edits you wish, regardless of size.
The reason there is a minimum character restriction is really for the sake of the reviewers of the edits - if everyone visiting the site was able to make whatever edits they require, big or small, then the suggested edit review queue would just get enormous and unmanageable. It's assumed (rightly or wrongly) that if you're making a substantial edit then that means it's worthwhile. OK, this means that on the occasions that minor edits that might improve or correct a post new users aren't going to be able to make such edits without adding more improvements to the post in addition to the spelling fixes.
Your options here are:
- See if there are any additional improvements you can make to that post in addition to the typo fix, sort those and submit that edit.
- Earn 2000 reputation so you can make the minor edits yourself (takes a bit longer to do, obviously, but think of it as a bit of motivation to up your reputation!)
- Speak to someone in the Chat room to get people in there to make the edit for you (probably the best option).
- Let the edit slide. It's just a typo, not the end of the world.
You're not alone in suggesting this requirement be removed, it's been discussed several times on the main StackExchange meta site - "Remove trivial edit restriction" so there's some good points made there too.