Bump tensorflow from 2.5.0 to 2.5.1 in /doc (#621)

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 2.5.0 to 2.5.1.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v2.5.0...v2.5.1)

---
updated-dependencies:
- dependency-name: tensorflow
  dependency-type: direct:production
...

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doc/requirements.txt

@@ -14,6 +14,6 @@ sphinx-autodoc-typehints==1.10.3
 sphinx-copybutton
 sphinx-automodapi
 sphinxcontrib-bibtex==0.4.2
-tensorflow==2.5.0
+tensorflow==2.5.1
 thewalrus>=0.16.0
 toml