Bump flask-restx from 0.4.0 to 0.5.1 in /examples/zoo_app

Bumps [flask-restx](https://github.com/python-restx/flask-restx) from 0.4.0 to 0.5.1.
- [Release notes](https://github.com/python-restx/flask-restx/releases)
- [Changelog](https://github.com/python-restx/flask-restx/blob/master/CHANGELOG.rst)
- [Commits](0.4.0...0.5.1)

---
updated-dependencies:
- dependency-name: flask-restx
  dependency-type: direct:production
...

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

examples/zoo_app/requirements.txt

@@ -2,7 +2,7 @@ aniso8601==9.0.1
 attrs==21.2.0
 click==7.1.2
 Flask==1.1.4
-flask-restx==0.4.0
+flask-restx==0.5.1
 itsdangerous==1.1.0
 Jinja2==2.11.3
 jsonschema==3.2.0