build(deps): bump pygments from 2.15.0 to 2.15.1 in /docs

Bumps [pygments](https://github.com/pygments/pygments) from 2.15.0 to 2.15.1.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.15.0...2.15.1)

---
updated-dependencies:
- dependency-name: pygments
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

docs/requirements.txt

@@ -9,7 +9,7 @@ imagesize==1.2.0
 Jinja2==2.11.3
 MarkupSafe==1.1.1
 packaging==20.9
-Pygments==2.15.0
+Pygments==2.15.1
 pyparsing==2.4.7
 pytz==2021.1
 recommonmark==0.7.1