small fix

SentenceAnagram.scala

//forked from https://gist.github.com/jkdeveyra/4030815

def sentenceAnagrams(sentence: Sentence): List[Sentence] = {

    def subSentence(occ: Occurrences): List[Sentence] = {
      if (occ.isEmpty) List(List())
      else
        for {
          x <- combinations(occ)
          y <- dictionaryByOccurrences.getOrElse(x, List())
          z <- subSentence(subtract(occ, x))
        } yield y :: z
    }

    subSentence(sentenceOccurrences(sentence))
  }