Bump tensorflow from 1.14.0 to 1.15.2 in /doc

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.14.0 to 1.15.2.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v1.14.0...v1.15.2)

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

Author: dependabot[bot]

doc/requirements.txt

@@ -1,4 +1,4 @@
 numpy >= 1.17.4
 networkx >= 2.1
 scipy >= 1.1.0
-tensorflow == 1.14.0
+tensorflow == 1.15.2