8241674 (XS): Fix incorrect jtreg option in FilePublisherPermsTest

[Julia Boes]

Hi,

Could someone please review this small fix of a recently pushed test? 
The option to extend the policy file has to be 'java.security.policy' 
instead of 'policy' -  this was discussed on this mailing list but then 
miraculously ignored by myself...

Bug: https://bugs.openjdk.java.net/browse/JDK-8241674

Webrev: http://cr.openjdk.java.net/~jboes/webrevs/8241674/webrev.00/

Regards,

Julia