qapi: Fix calc-dirty-rate example

The example shows {"command": ...}, which is wrong.  Fix it to
{"execute": ...}.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20220401082028.3583296-1-armbru@redhat.com>
Reviewed-by: Victor Toso <victortoso@redhat.com>

qapi/migration.json

@@ -1845,7 +1845,7 @@
 # Since: 5.2
 #
 # Example:
-#   {"command": "calc-dirty-rate", "arguments": {"calc-time": 1,
+#   {"execute": "calc-dirty-rate", "arguments": {"calc-time": 1,
 #                                                'sample-pages': 512} }
 #
 ##