Fix the value for the QEMU size parameter in the documentation

Instead of 1024m it shoud be 1024M. Using 1024m as value does not work:

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2014/10/27 10:21:41 packer-builder-qemu: 2014/10/27 10:21:41 Executing
/usr/bin/qemu-system-x86_64: []string{"-m", "1024m", "-redir",
"tcp:3213::22", "-device", "virtio-net,netdev=user.0", "-cdrom",
"/home/berendt/B1-Systems/openstack-appliance/packer_cache/436c246ce08d768bbacce00e39c11ecd09071565542788d08b3120de0228c9eb.iso",
"-machine", "type=pc-1.0,accel=kvm", "-vnc", "0.0.0.0:47", "-netdev",
"user,id=user.0", "-name", "openstack", "-drive",
"file=openstack/openstack.qcow2,if=virtio", "-boot", "once=d"}
2014/10/27 10:21:41 packer-builder-qemu: 2014/10/27 10:21:41 Started
Qemu. Pid: 2618
2014/10/27 10:21:41 packer-builder-qemu: 2014/10/27 10:21:41 Qemu
stderr: qemu-system-x86_64: -m 1024m: Parameter 'size' expects a size
==> qemu: Error launching VM: Qemu failed to start. Please run with logs
to get more info.
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