Bump nltk from 3.6.6 to 3.9 in /docs

Bumps [nltk](https://github.com/nltk/nltk) from 3.6.6 to 3.9.
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.6.6...3.9)

---
updated-dependencies:
- dependency-name: nltk
  dependency-type: direct:production
...

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

docs/requirements.txt

@@ -14,7 +14,7 @@ Markdown==3.3.4
 MarkupSafe==2.0.1
 mkdocs==1.3.0
 more-itertools==8.7.0
-nltk==3.6.6
+nltk==3.9
 packaging==20.9
 pluggy==0.13.1
 py==1.10.0