Bump pygments from 2.1.1 to 2.15.0 in /doc (haskell-servant/servant-quickcheck#90)

Bumps [pygments](https://github.com/pygments/pygments) from 2.1.1 to 2.15.0.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.1.1...2.15.0)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

servant-quickcheck/doc/requirements.txt

@@ -10,7 +10,7 @@ Jinja2==3.1.3
 livereload==2.4.1
 MarkupSafe==0.23
 pathtools==0.1.2
-Pygments==2.7.4
+Pygments==2.15.0
 pytz==2015.7
 PyYAML==3.11
 recommonmark==0.4.0