[Skiboot] [PATCH] README: Reflect openpower_mambo_defconfig name change

[Stewart Smith]

Reza Arbab <arbab at linux.ibm.com> writes:
> The name of this op-build config has changed. Use the new name.

Thanks, merged to master as of 46050485f5ef370cb0127835df12067bc36cbf9d

-- 
Stewart Smith
OPAL Architect, IBM.