fix: change the config name and fix typo in description of the number of retrieval executors (#13856)

api/configs/middleware/__init__.py

         default=False,
     )
 
-    RETRIEVAL_SERVICE_WORKER: NonNegativeInt = Field(
-        description="If True, enables the retrieval service worker.",
+    RETRIEVAL_SERVICE_EXECUTORS: NonNegativeInt = Field(
+        description="Number of processes for the retrieval service, default to CPU cores.",
         default=os.cpu_count(),
     )
 

api/core/rag/datasource/retrieval_service.py

 import concurrent.futures
 import json
+from concurrent.futures import ThreadPoolExecutor
 from typing import Optional
 
 from flask import Flask, current_app
         exceptions: list[str] = []
 
         # Optimize multithreading with thread pools
-        with concurrent.futures.ThreadPoolExecutor(max_workers=dify_config.RETRIEVAL_SERVICE_WORKER) as executor:  # type: ignore
+        with ThreadPoolExecutor(max_workers=dify_config.RETRIEVAL_SERVICE_EXECUTORS) as executor:  # type: ignore
             futures = []
             if retrieval_method == "keyword_search":
                 futures.append(