qapi: fix example of DUMP_COMPLETED event

Example output lacks mandatory member @timestamp.  Provide it.

Example output is not properly formatted. Fixing it by:
 - Adding '<-' to signalize it is receiving the data;
 - Adding extra spaces around members @result, @total and @completed

Signed-off-by: Victor Toso <victortoso@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-Id: <20220328140604.41484-6-victortoso@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

qapi/dump.json

@@ -161,9 +161,10 @@
 #
 # Example:
 #
-# { "event": "DUMP_COMPLETED",
-#   "data": {"result": {"total": 1090650112, "status": "completed",
-#                       "completed": 1090650112} } }
+# <- { "event": "DUMP_COMPLETED",
+#      "data": { "result": { "total": 1090650112, "status": "completed",
+#                            "completed": 1090650112 } },
+#      "timestamp": { "seconds": 1648244171, "microseconds": 950316 } }
 #
 ##
 { 'event': 'DUMP_COMPLETED' ,