Provide better code formating in dbimage-builder README
Provide better code formating in dbimage-builder README

Use code block for setup instructions as it includes commands

Change-Id: I0b17dea56cf24b338313fbaa8960aacfdc3b5e32

open-power-ref-design-toolkit/os-services,open-power-ref-design-toolkit/os-services,open-power-ref-design/os-services,open-power-ref-design-toolkit/os-services,open-power-ref-design/os-services