qapi: fix example of query-hotpluggable-cpus command

The example return type has the wrong member name. Fix it.

Problem was noticed when using the example as a test case for Go
bindings.

Signed-off-by: Victor Toso <victortoso@redhat.com>
Message-Id: <20220901085840.22520-10-victortoso@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

qapi/machine.json

@@ -958,9 +958,9 @@
 #
 # -> { "execute": "query-hotpluggable-cpus" }
 # <- {"return": [
-#      { "props": { "core": 8 }, "type": "POWER8-spapr-cpu-core",
+#      { "props": { "core-id": 8 }, "type": "POWER8-spapr-cpu-core",
 #        "vcpus-count": 1 },
-#      { "props": { "core": 0 }, "type": "POWER8-spapr-cpu-core",
+#      { "props": { "core-id": 0 }, "type": "POWER8-spapr-cpu-core",
 #        "vcpus-count": 1, "qom-path": "/machine/unattached/device[0]"}
 #    ]}'
 #