Bump tensorflow from 1.14.0 to 2.4.0 in /doc

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.14.0 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@v1.14.0...v2.4.0)

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

Author: dependabot[bot]

doc/requirements.txt

@@ -1,7 +1,7 @@
 numpy >= 1.17.4
 networkx >= 2.1
 scipy >= 1.1.0
-tensorflow == 1.14.0
+tensorflow == 2.4.0
 scikit-learn >= 0.22
 ogb_lite
 tqdm