Bump sphinx from 2.1.2 to 2.2.0 in /docs

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 2.1.2 to 2.2.0.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES)
- [Commits](sphinx-doc/sphinx@v2.1.2...v2.2.0)

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

docs/requirements.txt

@@ -1,7 +1,7 @@
 # This file is used to setup env
 # to generate documentation.
 
-sphinx==2.1.2
+sphinx==2.2.0
 sphinx_autodoc_typehints==1.7.0
 sphinxcontrib-mermaid==0.3.1
 recommonmark==0.6.0