Create and manage volumes in End User Guide: GB vs GiB

[Marc Morgan]

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 sentence "Size (GB): The size of the volume in gibibytes (GiB)." does not make any sense. If the unit is GiB, it should be "Size (GiB)". Maybe the error stretches beyond the documentation (e.g. to the UI).
- [ ] 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: 1.0.0 on 2019-08-16 16:57
SHA: a93ed3809b824edc455a8d848313d361fb93a4c7
Source: http://git.openstack.org/cgit/openstack/openstack-manuals/tree/doc/user-guide/source/dashboard_manage_volumes.rst
URL: https://docs.openstack.org/mitaka/user-guide/dashboard_manage_volumes.html