If someone writes in a manner such as "U hve to run daily ... only then U will lose weight" and you correct it to "You have to run daily, only then you will lose weight." I suppose you can't say you have corrected grammar. What can you say you have corrected? Punctuation?
I'd say you've corrected their writing style. They were writing in an informal slang style. It's been corrected to a more formal style.
I suppose you could say you've corrected the spelling, since "U" to mean "you" may not be in most dictionaries. It depends on if you see "U" as a slang word in its own right or as a misspelling of "you".