Bump werkzeug from 0.14.1 to 0.15.3 in /docs

Bumps [werkzeug](https://github.com/pallets/werkzeug) from 0.14.1 to 0.15.3.
- [Release notes](https://github.com/pallets/werkzeug/releases)
- [Changelog](https://github.com/pallets/werkzeug/blob/master/CHANGES.rst)
- [Commits](pallets/werkzeug@0.14.1...0.15.3)

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

Author: dependabot[bot]

docs/requirements.txt

@@ -212,7 +212,7 @@ vispy==0.5.3
 vlc==0.0.0
 wcwidth==0.1.7
 webencodings==0.5.1
-Werkzeug==0.14.1
+Werkzeug==0.15.3
 whaaaaat==0.5.2
 widgetsnbextension==3.2.1
 wrapt==1.10.11