update readme

Signed-off-by: junjie.jiang <junjie.jiang@zilliz.com>

README.md

@@ -18,20 +18,24 @@ Read data from sqlite or mysql.
 ### Example
 
 ```python
+
 from towhee import DataLoader, pipe, ops
 p = (
     pipe.input('image_path')
-    .map('image_path', 'image', ops.image_decode.cv2())
-    .map('image', 'vec', ops.image_embedding.timm(model_name='resnet50'))
-    .output('vec')
+        .map('image_path', 'image', ops.image_decode.cv2())
+            .map('image', 'vec', ops.image_embedding.timm(model_name='resnet50'))
+                .output('vec')
 
 )
 
-for data in DataLoader(ops.data_source.sql('sqlite:///./sqlite.db')):
+
+# table cols: id, image_path, label
+
+for data in DataLoader(ops.data_source.sql('sqlite:///./sqlite.db', 'image_table'), parser=lambda x: x[1]):
     print(p(data).to_list(kv_format=True))
 
 # batch
-for data in DataLoader(ops.data_source.sql('sqlite:///./sqlite.db'), batch_size=10):
+for data in DataLoader(ops.data_source.sql('sqlite:///./sqlite.db', 'image_table'), parser=lambda x: x[1], batch_size=10):
     p.batch(data)
 ```