qapi/migration.json: Remove a variable that doesn't exist in example

Remove the "active" variable in example for query-colo-status.
It is a doc bug from commit f56c0065

Signed-off-by: Zhang Chen <chen.zhang@intel.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190303145021.2962-6-chen.zhang@intel.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

qapi/migration.json

@@ -1393,7 +1393,7 @@
 # Example:
 #
 # -> { "execute": "query-colo-status" }
-# <- { "return": { "mode": "primary", "active": true, "reason": "request" } }
+# <- { "return": { "mode": "primary", "reason": "request" } }
 #
 # Since: 3.1
 ##