Bump nltk from 3.7 to 3.8 in /scripts/docs

Bumps [nltk](https://github.com/nltk/nltk) from 3.7 to 3.8.
- [Release notes](https://github.com/nltk/nltk/releases)
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.7...3.8)

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

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

scripts/docs/requirements.txt

@@ -1,3 +1,3 @@
-nltk==3.7
+nltk==3.8
 pyyaml==6.0
 Jinja2==3.1.2