[MLLIB] minor update to word2vec

very minor update Ishiihara

Author: Xiangrui Meng <meng@databricks.com>

Closes #2043 from mengxr/minor-w2v and squashes the following commits:

be649fd [Xiangrui Meng] remove map because we only need append
eccefcc [Xiangrui Meng] minor updates to word2vec

(cherry picked from commit 1870dbaa5591883e61b2173d064c1a67e871b0f5)
Signed-off-by: Xiangrui Meng <meng@databricks.com>