build(deps): bump pyyaml from 5.3.1 to 5.4 in /docs

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.3.1 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@5.3.1...5.4)

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

Author: dependabot[bot]

docs/requirements.txt

@@ -41,7 +41,7 @@ pygments==2.6.1
     # via mkdocs-material
 pymdown-extensions==7.1
     # via mkdocs-material
-pyyaml==5.3.1
+pyyaml==5.4
     # via mkdocs
 regex==2020.6.8
     # via nltk