repo delete prompts for mis-spelled repositories

I just ran `gh repo delete gh-bin-upgade` and then spent way too long trying to figure out why
it wasn't working, only to realize i was correctly spelling `gh-bin-upgrade` in the prompt.

This isn't a huge problem and I recognize that only prompting for real repositories means an
extra request, but it'd be nice to not be trying to type in a name that will never be
accepted.