docs/cpu-hotplug.rst: Fix rST markup issues

sphinx-build complains:

docs/cpu-hotplug.rst:67: ERROR: Unexpected indentation.
docs/cpu-hotplug.rst:69: ERROR: Unexpected indentation.
docs/cpu-hotplug.rst:74: WARNING: Block quote ends without a blank line; unexpected unindent.
docs/cpu-hotplug.rst:75: WARNING: Block quote ends without a blank line; unexpected unindent.
docs/cpu-hotplug.rst:76: SEVERE: Unexpected section title.

}
{
docs/cpu-hotplug.rst:78: WARNING: Block quote ends without a blank line; unexpected unindent.

These are the result of not indicating one of the literal
blocks by finishing the preceding paragraph with the "::" marker.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-id: 20190305172139.32662-2-peter.maydell@linaro.org
Message-id: 20190228145624.24885-2-peter.maydell@linaro.org

docs/cpu-hotplug.rst

@@ -60,7 +60,7 @@ vCPU hotplug
     hot-plugged (no "qom-path" member).  From its output in step (3), we
     can see that ``IvyBridge-IBRS-x86_64-cpu`` is present in socket 0,
     while hot-plugging a CPU into socket 1 requires passing the listed
-    properties to QMP ``device_add``:
+    properties to QMP ``device_add``::
 
       (QEMU) device_add id=cpu-2 driver=IvyBridge-IBRS-x86_64-cpu socket-id=1 core-id=0 thread-id=0
       {