Bump tensorflow from 2.0.3 to 2.4.0 in /doc (#552)

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 2.0.3 to 2.4.0.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v2.0.3...v2.4.0)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Josh Izaac <josh146@gmail.com>

doc/requirements.txt

@@ -13,5 +13,5 @@ sphinx-autodoc-typehints==1.10.3
 sphinx-copybutton
 sphinx-automodapi
 sphinxcontrib-bibtex==0.4.2
-tensorflow==2.2.0
+tensorflow==2.4.0
 toml