Update doc update ms.date metadata

@JoanneHendrickson please update the metadata reflecting when the doc was updated when submitting changes in the future

Author: andrewconnell

docs/apis/migration-api-overview.md

@@ -1,6 +1,6 @@
 ---
 title: "SharePoint Online Import Migration API"
-ms.date: 05/08/2020
+ms.date: 06/14/2020
 ms.author: jhendr
 author: JoanneHendrickson
 manager: pamgreen