RFR: 8183901 : Fix broken links to "Package Sealing" in the JAR spec

[Brent Christian]

Hi,

Please review my fix of a few broken links to the "Package Sealing" 
section of the Jar Spec[1].

http://cr.openjdk.java.net/~bchristi/8183901/

Thanks,
-Brent

1. https://docs.oracle.com/javase/9/docs/specs/jar/jar.html#package-sealing