Fix title of env vars section of redhat-common README to match template
Fix title of env vars section of redhat-common README to match template

Change-Id: I37b33eba8af507c03088bc0025280a696e85b04f

openstack/diskimage-builder,takahashinobuyuki/diskimage-builder,switch-ch/diskimage-builder,zioproto/diskimage-builder,takahashinobuyuki/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,openstack/diskimage-builder,zioproto/diskimage-builder,switch-ch/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder