fix(deps): incorrect nltk download dir (#7447)

### What problem does this PR solve?

Fix https://github.com/infiniflow/ragflow/issues/7224 and
https://github.com/infiniflow/ragflow/issues/6793

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)a
- [x] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

docker/launch_backend_service.sh

@@ -47,6 +47,9 @@ STOP=false
 # Array to keep track of child PIDs
 PIDS=()
 
+# Set the path to the NLTK data directory
+export NLTK_DATA="./nltk_data"
+
 # Function to handle termination signals
 cleanup() {
   echo "Termination signal received. Shutting down..."