RFR(XS): 8200469: Update link to license in Docs.gmk

[Iris Clark]

Please review this small change to update the link to the
Specification license referenced by the JavaDoc API.  To
avoid the need for future updates based on the version
number, we should use $(VERSION_NUMBER) instead of a
specific version number.

Bug:

    8200469: Update link to license in Docs.gmk
    https://bugs.openjdk.java.net/browse/JDK-8200469

webrev:

    http://cr.openjdk.java.net/~iris/8200469/webrev/

Thanks,
Iris