Bump urllib3 from 1.26.17 to 1.26.18 in /docs

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.26.17 to 1.26.18.
- [Release notes](https://github.com/urllib3/urllib3/releases)
- [Changelog](https://github.com/urllib3/urllib3/blob/main/CHANGES.rst)
- [Commits](urllib3/urllib3@1.26.17...1.26.18)

---
updated-dependencies:
- dependency-name: urllib3
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

docs/requirements.txt

@@ -15,5 +15,5 @@ snowballstemmer==1.2.1
 Sphinx==1.5.1
 sphinx-rtd-theme==0.1.9
 sphinxcontrib-phpdomain==0.2.1
-urllib3==1.26.17
+urllib3==1.26.18
 cakephp-theme