doc: qemu_virt: Fix doc styles

Remove the unnecessary blank line at the end of the doc.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>

docs/platform/qemu_virt.md

@@ -149,4 +149,3 @@ qemu-system-riscv32 -M virt -m 256M -nographic \
 	-device virtio-blk-device,drive=hd0 \
 	-append "root=/dev/vda rw console=ttyS0"
 ```
-