There’s only one way you can spell the adverb preferably. You can’t add another “f,” “r,” or “l”—there’s really no need to do it.
Let’s be honest here—mistakes happen to the best of us. We’d have a hard time finding a writer who, at some point, didn’t miswrite “the” as “hte” or “teh.” In haste, it might also be possible to mistake “to” for “too,” or “their” for “they’re.” And that’s perfectly fine, as long as you go over your work, notice your mistakes, and fix them.