qemu-options.hx: Fix formatting of -machine memory-backend option

The documentation of the -machine memory-backend has some minor
formatting errors:
 * Misindentation of the initial line meant that the whole option
   section is incorrectly indented in the HTML output compared to
   the other -machine options
 * The examples weren't indented, which meant that they were formatted
   as plain run-on text including outputting the "::" as text.
 * The a) b) list has no rst-format markup so it is rendered as
   a single run-on paragraph

Fix the formatting.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-id: 20210719105257.3599-1-peter.maydell@linaro.org

qemu-options.hx

@@ -104,19 +104,23 @@ SRST
 
         For example:
         ::
+
             -object memory-backend-file,id=pc.ram,size=512M,mem-path=/hugetlbfs,prealloc=on,share=on
             -machine memory-backend=pc.ram
             -m 512M
 
         Migration compatibility note:
-        a) as backend id one shall use value of 'default-ram-id', advertised by
+
+        * as backend id one shall use value of 'default-ram-id', advertised by
           machine type (available via ``query-machines`` QMP command), if migration
           to/from old QEMU (<5.0) is expected.
-        b) for machine types 4.0 and older, user shall
+        * for machine types 4.0 and older, user shall
           use ``x-use-canonical-path-for-ramblock-id=off`` backend option
           if migration to/from old QEMU (<5.0) is expected.
+
         For example:
         ::
+
             -object memory-backend-ram,id=pc.ram,size=512M,x-use-canonical-path-for-ramblock-id=off
             -machine memory-backend=pc.ram
             -m 512M