Update README

Signed-off-by: shiyu22 <shiyu.chen@zilliz.com>

README.md

@@ -10,7 +10,9 @@ This Operator generates feature vectors from the pytorch pretrained **Resnet50**
 
 ## Interface
 
-`__init__(self, model_name: str, framework: str = 'pytorch')`
+```python
+__init__(self, model_name: str, framework: str = 'pytorch')
+```
 
 **Args:**
 
@@ -21,7 +23,9 @@ This Operator generates feature vectors from the pytorch pretrained **Resnet50**
   - the framework of the model
   - supported types: `str`, default is 'pytorch'
 
-`__call__(self, img_tensor: torch.Tensor)`
+```python
+__call__(self, img_tensor: torch.Tensor)
+```
 
 **Args:**