Update centralized-rbac.rst

[renjolras-splunk]

Move of the "30 minutes" note from end of chapter to section 2

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Describe the change

Note "It takes around 30 minutes for a new role to be available on the Roles page." not well positioned.
Note rephrased and moved to the section 2 where the related roles are created.

[aurbiztondo-splunk]

Thanks for the updates, @renjolras-splunk!