Increase last supported opset to 19 in readme (onnx#1087)

See: https://github.com/onnx/sklearn-onnx/blob/d2029c1a9752f62a63fc5c4447b4d9fe75e8fe39/skl2onnx/__init__.py#L12

Signed-off-by: XiangRongLin <41164160+XiangRongLin@users.noreply.github.com>
Signed-off-by: Xavier Dupre <xadupre@microsoft.com>

README.md

@@ -18,7 +18,7 @@ including models or transformers coming from external libraries.
 ## Documentation
 Full documentation including tutorials is available at [https://onnx.ai/sklearn-onnx/](https://onnx.ai/sklearn-onnx/).
 [Supported scikit-learn Models](https://onnx.ai/sklearn-onnx/supported.html)
-Last supported opset is 15.
+Last supported opset is 19.
 
 You may also find answers in [existing issues](https://github.com/onnx/sklearn-onnx/issues?utf8=%E2%9C%93&q=is%3Aissue)
 or submit a new one.