build(deps): bump nltk from 3.5 to 3.8.1 in /docs

Bumps [nltk](https://github.com/nltk/nltk) from 3.5 to 3.8.1.
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.5...3.8.1)

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

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

docs/requirements.txt

@@ -25,25 +25,25 @@ markdown==3.2.2
     #   pymdown-extensions
 markupsafe==1.1.1
     # via jinja2
-mkdocs-material-extensions==1.0
-    # via mkdocs-material
-mkdocs-material==5.5.12
-    # via
-    #   -r requirements.in
-    #   mkdocs-material-extensions
 mkdocs==1.1.2
     # via
     #   -r requirements.in
     #   mkdocs-material
-nltk==3.5
+mkdocs-material==5.5.12
+    # via
+    #   -r requirements.in
+    #   mkdocs-material-extensions
+mkdocs-material-extensions==1.0
+    # via mkdocs-material
+nltk==3.8.1
     # via lunr
 pygments==2.7.4
     # via mkdocs-material
 pymdown-extensions==7.1
     # via mkdocs-material
 pyyaml==5.4
     # via mkdocs
-regex==2020.6.8
+regex==2024.5.15
     # via nltk
 six==1.15.0
     # via