OvmfPkg/README: refer to MdeModulePkg & PlatformBootManagerLib in examples

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>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>