Bump nltk from 3.6.2 to 3.6.4

Bumps [nltk](https://github.com/nltk/nltk) from 3.6.2 to 3.6.4.
- [Release notes](https://github.com/nltk/nltk/releases)
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.6.2...3.6.4)

---
updated-dependencies:
- dependency-name: nltk
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]