update readme

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

README.md

@@ -18,6 +18,7 @@ Read data from sqlite or mysql.
 ### Example
 
 ```python
+
 from towhee import DataLoader, pipe, ops
 p = (
     pipe.input('image_path')
@@ -27,11 +28,14 @@ p = (
 
 )
 
-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)
 ```