Bump pygments from 2.2.0 to 2.7.4 in /docs

Bumps [pygments](https://github.com/pygments/pygments) from 2.2.0 to 2.7.4.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.2.0...2.7.4)

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

Author: dependabot[bot]

docs/requirements.txt

@@ -57,7 +57,7 @@ prompt-toolkit==1.0.15
 ptyprocess==0.5.2
 py==1.4.34
 pycparser==2.18
-Pygments==2.2.0
+Pygments==2.7.4
 pylint==1.7.2
 pyparsing==2.2.0
 PySocks==1.6.7