fix: correct typo in ETL type comment in .env.example (#10822)

docker/.env.example

@@ -562,7 +562,7 @@ UPLOAD_FILE_SIZE_LIMIT=15
 # The maximum number of files that can be uploaded at a time, default 5.
 UPLOAD_FILE_BATCH_LIMIT=5
 
-# ETl type, support: `dify`, `Unstructured`
+# ETL type, support: `dify`, `Unstructured`
 # `dify` Dify's proprietary file extraction scheme
 # `Unstructured` Unstructured.io file extraction scheme
 ETL_TYPE=dify