This is just wrong, both names are accepted and "Littorio" is the most used, also because she was supposedly laid down earlier(even if on the same day) and was supposed to be the leader of the class. Even Bagnasco and De Toro literally called their book " The Littorio Class: Italy's Last and Largest Battleships 1937–1948 ".
In addition to that, even this official Marina Militare site calls them as such: http://www.regiamarina.net/detail_text_with_list.asp?nid=70&lid=2.