Fix tutorials/keras-resnet50.ipynb link in the README.md (#1566)

Signed-off-by: Kentaro Iizuka <[email protected]>

Author: iizukak

README.md

@@ -309,7 +309,7 @@ model_proto, external_tensor_storage = tf2onnx.convert.from_keras(model,
         An ONNX model_proto and an external_tensor_storage dict.
 ```
 
-See [tutorials/keras-resnet50.ipynb](tutorials/keras-resnet50) for an end to end example.
+See [tutorials/keras-resnet50.ipynb](tutorials/keras-resnet50.ipynb) for an end to end example.
 
 ### from_function (tf-2.0 and newer)
 ```