The problem is that most sheds have too flimsy a frame to accept the point loads from a bracing wire. A ratchet strap, by the way, is the best bracing you can find for this circumstance........but as I say, getting a decent fixing at either end, something which won't just break out under tension, is going to be your issue.
The next thing to understand is that simply stretching it back into rectinlinearity is only half the problem. You then have to retain it like that. A diagonal brace nailed or screwed to each vertical in the frame would do the job nicely (a piece of 25x100 is best, because you can get 2 fixings per stud), but the other option is some ply or OSB planted onto the face of the frame and screwed in place.