Bump pygments from 2.4.2 to 2.7.4 in /docs

Bumps [pygments](https://github.com/pygments/pygments) from 2.4.2 to 2.7.4.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.4.2...2.7.4)

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

Author: dependabot[bot]

docs/requirements.txt

@@ -10,7 +10,7 @@ Jinja2==2.11.3
 MarkupSafe==1.1.1
 packaging==19.2
 pbr==5.4.3
-Pygments==2.4.2
+Pygments==2.7.4
 pyparsing==2.4.2
 pytz==2019.3
 requests==2.22.0