Example of How to work with asynchronous actions in Mistral is incorrect

[Marc Carmier]

This bug tracker is for errors with the documentation, use the following as a template and remove or add fields as you see fit. Convert [ ] into [x] to check boxes:

- [x] This doc is inaccurate in this way: The section "Pushing action result to Mistral" show a raw call POST <Mistral-Callback-URL>/v2/action-executions/<id> but the real call must be a PUT and <Mistral-Callback-URL> does not contains the base URL of the server but /v2/action-executions/<id>
The call should be PUT <mistral API URL>/<Mistral-Callback-URL>

- [ ] This is a doc addition request.
- [ ] I have a fix to the document that I can paste below including example: input and output.

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-----------------------------------
Release: 10.1.0.dev44 on 2020-01-24 12:10:44
SHA: 63225d41e41dbedc341f2ef0724901f62dcd5d8b
Source: https://opendev.org/openstack/mistral/src/doc/source/user/asynchronous_actions.rst
URL: https://docs.openstack.org/mistral/latest/user/asynchronous_actions.html