Bump tensorflow from 1.15.4 to 2.4.0 in /study_material/intro-to-rnns

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.15.4 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@v1.15.4...v2.4.0)

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

study_material/intro-to-rnns/requirements.txt

@@ -34,7 +34,7 @@ pyzmq==16.0.2
 qtconsole==4.3.0
 simplegeneric==0.8.1
 six==1.10.0
-tensorflow==1.15.4
+tensorflow==2.4.0
 terminado==0.6
 testpath==0.3
 tornado==4.4.3