examples: use punkt_tab instead of punkt (#6907)

This was causing an error since we depend on punkt_tab.

examples/python-grounded-factuality-rag-check/main.py

@@ -9,7 +9,7 @@ import nltk
 warnings.filterwarnings(
     "ignore", category=FutureWarning, module="transformers.tokenization_utils_base"
 )
-nltk.download("punkt", quiet=True)
+nltk.download("punkt_tab", quiet=True)
 
 
 def getArticleText(url):