OvmfPkg/README: refer to MdeModulePkg & PlatformBootManagerLib in examples
    
    The "UNIXGCC Debug" section happens to name PlatformBdsLib and
    IntelFrameworkModulePkg's BdsDxe as examples. OVMF will soon stop offering
    those even as a fallback option.
    
    Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
    Cc: Gary Ching-Pang Lin <glin@suse.com>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Ruiyu Ni <ruiyu.ni@intel.com>
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>