Update show_topics method from Mallet wrapper #1747
Description
Need to update https://github.com/RaRe-Technologies/gensim/blob/3.1.0/gensim/models/wrappers/ldamallet.py#L250 (replace num_words to topn) to prevent warning (report comes from https://groups.google.com/forum/#!topic/gensim/r3YIaqPRmzU)