Keystone doc has wrong keystone-manage command

Fixed the keystone-manage command (--extension should be placed after db_sync)

Amend: making it more readable by seperating command from text

Change-Id: Iaf8dcacaa38cdcbaa867bb6e374a87c00cd45ac3
backport: none
Closes-bug: 1281819
Merge "Keystone doc has wrong keystone-manage command"