Bump tensorflow in /examples/python_bert_squad_example (#834)

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 2.4.0 to 2.5.0.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](https://github.com/tensorflow/tensorflow/compare/v2.4.0...v2.5.0

)
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examples/python_bert_squad_example/requirements_bertsquad.txt

-tensorflow==2.4.0
+tensorflow==2.5.0
 onnxruntime
 tokenizers
\ No newline at end of file