docs: Format literals correctly

In rST markup, single backticks `like this` represent "interpreted
text", which can be handled as a bunch of different things if tagged
with a specific "role":
https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#interpreted-text
(the most common one for us is "reference to a URL, which gets
hyperlinked").

The default "role" if none is specified is "title_reference",
intended for references to book or article titles, and it renders
into the HTML as <cite>...</cite> (usually comes out as italics).

This commit fixes various places in the manual which were
using single backticks when double backticks (for literal text)
were intended, and covers those files where only one or two
instances of these errors were made.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

docs/system/riscv/microchip-icicle-kit.rst

@@ -95,7 +95,7 @@ Then we can boot the machine by:
       -serial chardev:serial1
 
 With above command line, current terminal session will be used for the first
-serial port. Open another terminal window, and use `minicom` to connect the
+serial port. Open another terminal window, and use ``minicom`` to connect the
 second serial port.
 
 .. code-block:: bash

docs/system/riscv/virt.rst

@@ -1,7 +1,7 @@
 'virt' Generic Virtual Platform (``virt``)
 ==========================================
 
-The `virt` board is a platform which does not correspond to any real hardware;
+The ``virt`` board is a platform which does not correspond to any real hardware;
 it is designed for use in virtual machines. It is the recommended board type
 if you simply want to run a guest such as Linux and do not care about
 reproducing the idiosyncrasies and limitations of a particular bit of