fix rag-service requirements (#2843)

In response to the issue raised in # 281, I have added a dependency of
'nbconvert' to the Python dependency list of the avaent rag server,
hoping to fix the problem

py/rag-service/requirements.txt

@@ -177,3 +177,4 @@ websockets==14.2
 wrapt==1.17.2
 yarl==1.18.3
 zipp==3.21.0
+nbconvert==7.16.6