When I first saw this, I thought of the Italian meaning “first” or “before” which would follow the English fascination of Italian names and culture that Shakespeare uses so often in his works (this Folio was published in London). Then, when searching “prima” on the OED, it has come to mean “The page of printer’s copy on which a new sheet begins and on which the first word of the sheet is marked” (“† prima, n.1.”). Perhaps this usage of the term is what later (circa 1800s) became common usage for proofs and publication.