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>

docs/system/riscv/virt.rst

@@ -168,14 +168,19 @@ Enabling TPM
 
 A TPM device can be connected to the virt board by following the steps below.
 
-First launch the TPM emulator
+First launch the TPM emulator:
 
-    swtpm socket --tpm2 -t -d --tpmstate dir=/tmp/tpm \
+.. code-block:: bash
+
+  $ swtpm socket --tpm2 -t -d --tpmstate dir=/tmp/tpm \
         --ctrl type=unixio,path=swtpm-sock
 
-Then launch QEMU with:
+Then launch QEMU with some additional arguments to link a TPM device to the backend:
 
-    ...
+.. code-block:: bash
+
+  $ qemu-system-riscv64 \
+    ... other args .... \
     -chardev socket,id=chrtpm,path=swtpm-sock \
     -tpmdev emulator,id=tpm0,chardev=chrtpm \
     -device tpm-tis-device,tpmdev=tpm0