The revised version of the sentence would be as follows: Mark Twain, who was born in Florida, is one of America's most famous authors.
Explanation:
A nonrestrictive element is nonessential because you don't need the information it provides. These elements are always separated with commas from the rest of the sentence to indicate that they are not essential. If they were dropped, the meaning of the sentence would be left unchanged (1). In the sentence above, the nonrestrictive element is a nonessential relative clause.
1) Mark Twain is one of America's most famous authors.
