docs(go extractor): remove mention of kindex and indexpack (#3939)
docs(go extractor): remove mention of kindex and indexpack (#3939)

the extractor only handles kzip
kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe