docs/system/target-riscv.rst: tidy CPU firmware section

This is how the content of the "RISC-V CPU firmware" section is
displayed after the html is generated:

"When using the sifive_u or virt machine there are three different
firmware boot options: 1. -bios default - This is the default behaviour
if no -bios option is included. (...) 3. -bios <file> - Tells QEMU to
load the specified file as the firmware."

It's all in the same paragraph, in a numbered list, and no special
formatting for the options.

Tidy it a bit by adding line breaks between items and its description.
Remove the numbered list. And apply formatting for the options cited in
the middle of the text.

Cc: qemu-trivial@nongnu.org
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230712143728.383528-1-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

docs/system/target-riscv.rst

@@ -76,11 +76,19 @@ RISC-V CPU firmware
 
 When using the ``sifive_u`` or ``virt`` machine there are three different
 firmware boot options:
-1. ``-bios default`` - This is the default behaviour if no -bios option
-is included. This option will load the default OpenSBI firmware automatically.
-The firmware is included with the QEMU release and no user interaction is
-required. All a user needs to do is specify the kernel they want to boot
-with the -kernel option
-2. ``-bios none`` - QEMU will not automatically load any firmware. It is up
-to the user to load all the images they need.
-3. ``-bios <file>`` - Tells QEMU to load the specified file as the firmware.
+
+* ``-bios default``
+
+This is the default behaviour if no ``-bios`` option is included. This option
+will load the default OpenSBI firmware automatically. The firmware is included
+with the QEMU release and no user interaction is required. All a user needs to
+do is specify the kernel they want to boot with the ``-kernel`` option
+
+* ``-bios none``
+
+QEMU will not automatically load any firmware. It is up to the user to load all
+the images they need.
+
+* ``-bios <file>``
+
+Tells QEMU to load the specified file as the firmware.