Fix link for testpypi (#39360)

fix link

setup.py

@@ -52,7 +52,7 @@ To create the package for pypi.
    twine upload dist/* -r testpypi --repository-url=https://test.pypi.org/legacy/
 
    Check that you can install it in a virtualenv by running:
-   pip install -i https://testpypi.python.org/pypi transformers
+   pip install -i https://test.pypi.org/simple/ transformers
 
    Check you can run the following commands:
    python -c "from transformers import pipeline; classifier = pipeline('text-classification'); print(classifier('What a nice release'))"