Bump pygments from 2.7.4 to 2.15.0 in /docs

Bumps [pygments](https://github.com/pygments/pygments) from 2.7.4 to 2.15.0.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.7.4...2.15.0)

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

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

docs/requirements.txt

@@ -7,7 +7,7 @@ idna==2.7
 imagesize==0.7.1
 Jinja2==2.11.3
 MarkupSafe==0.23
-Pygments==2.7.4
+Pygments==2.15.0
 pytz==2018.7
 requests==2.20.1
 six==1.11.0