qemu-ppc: fixup wrong startup example, use either -hda or -drive

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

board/qemu/ppc-g3beige/readme.txt

@@ -1,6 +1,6 @@
 Run the emulation with:
 
-  qemu-system-ppc -M g3beige -kernel output/images/vmlinux -hda -drive file=output/images/rootfs.ext2,format=raw -append "console=ttyS0 root=/dev/hda" -serial stdio -net nic,model=rtl8139 -net user
+  qemu-system-ppc -M g3beige -kernel output/images/vmlinux -drive file=output/images/rootfs.ext2,format=raw -append "console=ttyS0 root=/dev/hda" -serial stdio -net nic,model=rtl8139 -net user
 
 The login prompt will appear in the terminal that started Qemu. The
 graphical window is the framebuffer.