Update links to the nltk book

[dannysepler]

Closes #2891

[stevenbird]

Thanks @dannysepler

[tomaarsen]

For me, the old links of https://www.nltk.org/book work on both Firefox and Chrome, and redirect to https://www.nltk.org/book/.

The links that fail for me (on Firefox) are:

http://nltk.org/book
https://nltk.org/book

The following links work:

https://www.nltk.org/book
https://www.nltk.org/book/
www.nltk.org/book/
www.nltk.org/book

I can't really explain it. It's definitely something to do with the inclusion of www. That said, this PR can be merged without issues.

[dannysepler]

You're right @tomaarsen! Those two links also don't work for me, whereas the other links all work for me. So I'm guessing this PR didn't actually do anything (oops) and the actually issue isn't closed yet. Rather, it has more to do with links on the website rather than in the nltk repository itself

I opened this in the nltk.github.com repository instead nltk/nltk.github.com#20