Bump urllib3 from 1.25.3 to 1.25.8 in /bin/transformers/docs

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.25.3 to 1.25.8.
- [Release notes](https://github.com/urllib3/urllib3/releases)
- [Changelog](https://github.com/urllib3/urllib3/blob/main/CHANGES.rst)
- [Commits](urllib3/urllib3@1.25.3...1.25.8)

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

Author: dependabot[bot]

bin/transformers/docs/requirements.txt

@@ -25,7 +25,7 @@ sphinxcontrib-htmlhelp==1.0.2
 sphinxcontrib-jsmath==1.0.1
 sphinxcontrib-qthelp==1.0.2
 sphinxcontrib-serializinghtml==1.1.3
-urllib3==1.25.3
+urllib3==1.25.8
 sphinx-markdown-tables==0.0.9
 numpy==1.17.2
 tensorflow==2.0.0rc2