Bump pygments from 2.5.2 to 2.7.4 in /docs

Bumps [pygments](https://github.com/pygments/pygments) from 2.5.2 to 2.7.4.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.5.2...2.7.4)

---
updated-dependencies:
- dependency-name: pygments
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

docs/requirements.txt

@@ -11,7 +11,7 @@ Jinja2==2.11.1
 MarkupSafe==1.1.1
 packaging==20.1
 pbr==5.4.4
-Pygments==2.5.2
+Pygments==2.7.4
 pyparsing==2.4.6
 pytz==2019.3
 requests==2.23.0