Bump nltk in /nemo/retriever-synthetic-data-generation

Bumps [nltk](https://github.com/nltk/nltk) from 3.8.1 to 3.9.4.
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.8.1...3.9.4)

---
updated-dependencies:
- dependency-name: nltk
  dependency-version: 3.9.4
  dependency-type: direct:production
...

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

Author: dependabot[bot]

nemo/retriever-synthetic-data-generation/requirements.txt

@@ -4,7 +4,7 @@ pydantic==2.6.4
 tqdm==4.66.2
 transformers==4.42.1
 hydra-core==1.3.2
-nltk==3.8.1
+nltk==3.9.4
 beir==2.0.0
 sentence-transformers==3.0.1
 jupyter==1.0.0