centos in diskimage-builder example variable is wrong

[Gudrutis]

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

After setting DIB_DISTRIBUTION_MIRROR variable, diskimage-builder prepends 'centos/{version}..',
for example:

DIB_DISTRIBUTION_MIRROR=http://linuxsoft.cern.ch/cern/centos

yum will try to install from `http://linuxsoft.cern.ch/cern/centoscentos/{version}
in my use case, I had to set DIB_DISTRIBUTION_MIRROR=http://linuxsoft.cern.ch/cern/ for it to work.

DIB_DISTRIBUTION_MIRROR=http://linuxsoft.cern.ch/cern/

I suggest in the example to change `DIB_DISTRIBUTION_MIRROR=http://amirror.com/centos` to DIB_DISTRIBUTION_MIRROR=http://amirror.com/

Or give more clarification:
```
If YUM repo is located at http://amirror.com/centos/{version}, set
DIB_DISTRIBUTION_MIRROR=http://amirror.com/

-----------------------------------
Release: 2.33.1.dev18 on 2020-02-26 03:09:33
SHA: fa6574715cdd134857a299341d3f00dc77eddd6b
Source: https://opendev.org/openstack/diskimage-builder/src/doc/source/elements/centos/README.rst
URL: https://docs.openstack.org/diskimage-builder/latest/elements/centos/README.html