docs/system: clean up code escape for riscv virt platform

docs/system: clean up code escape for riscv virt platform

The example code is rendered slightly mangled due to missing code
block. Properly escape the code block and add shell prompt and qemu to
fit in with the other examples on the page.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20220905163939.1599368-1-alex.bennee@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>