Minor improvements in dbimage-builder README
Minor improvements in dbimage-builder README

Better wording for Getting Started section

Change-Id: I27caf68b7e6cf628d01835d99f786eecc9fec5df

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