Improve the error message in middleware

Author: seratch

slack_bolt/middleware/middleware_error_handler.py

@@ -63,5 +63,5 @@ def handle(
         request: BoltRequest,
         response: Optional[BoltResponse],
     ):
-        message = f"Failed to run a middleware middleware (error: {error})"
+        message = f"Failed to run a middleware (error: {error})"
         self.logger.exception(message)