Update 07_sentiment_with_doc2vec.py

[RomanGurinovich]

When run on custom text file with vocab len != 7500, close_word = word_dictionary_rev[nearest[k]] throws KeyValue error.

Looking in the range of the len(word_dictionary_rev)]) generalizes the code.

[nfmcclure]

Ah yes, thanks for this. I'll merge this in the next week or so. I want to keep this active as a reminder that I should fix the jupyter notebook as well.