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

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>