Bump pygments from 2.9.0 to 2.15.0 in /docs

Bumps [pygments](https://github.com/pygments/pygments) from 2.9.0 to 2.15.0.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.9.0...2.15.0)

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

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

docs/requirements.txt

@@ -11,7 +11,7 @@ jinja2==3.0.1
 markdown==3.3.4
 markupsafe==2.0.1
 packaging==20.9
-pygments==2.9.0
+pygments==2.15.0
 pyparsing==2.4.7
 pytz==2021.1
 recommonmark==0.6.0