I accidentally spelled “world” as “word” and didn’t realize until after I submitted my application to UNC. Is this a big deal? Should I do something?
IMO it’s not a big deal, as long as the sentence still makes sense in-context of the entire essay. Just make sure to proofread your future applications more thoroughly. 
It is not a big deal. Small typos happen.
Now, the poster on another thread currently on CC who put in the name of the wrong university in a short answer essay might have reason to be more worried! but even there it’s not an absolute dealbreaker.
Don’t sweat it. Definitely don’t do anything like reaching out to your admission officer or trying to send in something. Admission officers are flying through your application–they probably won’t even catch it. (But a good lesson: I’ve found for important writing, print it out and read it out loud. You’ll catch those world/word nits.)
–MCS