qapi: fix example of BLOCK_IO_ERROR event

Example output lacks mandatory member @reason.  Provide it.
Signed-off-by: NVictor Toso <victortoso@redhat.com>
Message-Id: <20220328140604.41484-4-victortoso@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>

qapi/block-core.json

@@ -5058,7 +5058,8 @@
 #      "data": { "device": "ide0-hd1",
 #                "node-name": "#block212",
 #                "operation": "write",
-#                "action": "stop" },
+#                "action": "stop",
+#                "reason": "No space left on device" },
 #      "timestamp": { "seconds": 1265044230, "microseconds": 450486 } }
 #
 ##