[16] RFR (XS) 8246803: Update link to license in Docs.gmk

[Iris Clark]

Hi.

Please review this small change to update the link to the Specification license referenced
by the JavaDoc API for JDK 16:

bug:

    8246803: Update link to spec license in Docs.gmk
    https://bugs.openjdk.java.net/browse/JDK-8246803

webrev:

    https://cr.openjdk.java.net/~iris/8246803/webrev/

Due to changes in the server hosting the Spec license redirect, we must use a new URL
in JDK 16 which omits the "technetwork" token.  The following link for 16 is now live:

    https://www.oracle.com/java/javase/terms/license/java16speclicense.html

It should correspond to the new link generated at JDK 16 build time  The link for
JDK 15 is unchanged.

Thanks,
Iris