Bump pygments from 2.1 to 2.7.4 in /docs

Bumps [pygments](https://github.com/pygments/pygments) from 2.1 to 2.7.4.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.1...2.7.4)

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

Author: dependabot[bot]

docs/requirements.txt

@@ -3,7 +3,7 @@ Babel==2.2.0
 docutils==0.12
 Jinja2==2.10.1
 MarkupSafe==0.23
-Pygments==2.1
+Pygments==2.7.4
 pytz==2015.7
 six==1.10.0
 snowballstemmer==1.2.1