>>1854373The japanese sound for the mora ロ in her name (ホロ) is neither RO nor LO but something in between, so either could be a correct way of spelling her name in latin letters. (Though technically, neither is correct.)
Either way the author wrote that it should be Holo, and she signs her name as (a very stylized) Holo in the anime.
But in the end, call her what you will, just don't say "Horo" is correct with nothing to back it up except for the early fan translations.