NLP IMDB link is broken in readme.md

[DawnStone]

The https://github.com/NervanaSystems/ModelZoo/tree/master/NLP/readme.md file includes a link to:

<
[IMDB](./IMDB)

, but the link is broken and causes a 404 File not found error.

The link should be updated to the updated location at:

<
[IMDB](./SentimentClassification/IMDB)

the header formatting also should be set to match the first section.

<

Sentiment classification

IMDB

should be:

<

Sentiment classification

IMDB