Docu: aodh shell utility - Wrong Alarm Creation

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:
1. Creation of alarm with metric --metric cpu_util is not possible since Stein. Gnoochi does not contain this metric anymore.
2. --resource_id <RES_ID> --resource_type generic --aggregation_method have to be written with hyphen not underscore:
--resource-id <RES_ID> --resource-type generic --aggregation-method
- [ ] 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: on 2018-06-13 15:30:49
SHA: a8651ec642058a17cd7a6b3648d8250450a73efb
Source: https://opendev.org/openstack/python-aodhclient/src/doc/source/shell.rst
URL: https://docs.openstack.org/python-aodhclient/latest/shell.html