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

[Tim Bell]

Iris:

> 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/

Looks good.

Tim