Bump nltk from 3.8.1 to 3.9 in /backend/embedding (aws-samples#495)

* Bump nltk from 3.8.1 to 3.9 in /backend/embedding

Bumps [nltk](https://github.com/nltk/nltk) from 3.8.1 to 3.9.
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.8.1...3.9)

---
updated-dependencies:
- dependency-name: nltk
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>

* Bump nltk from 3.9 to 3.9.1 in /backend/embedding

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Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Yukinobu Mine <[email protected]>