Update use of "-Djava.security.debug=help" option

Recent startup performance improvements have been made such that running "java -Djava.security.debug=help" (without specifying an application to run) no longer works (see JDK-8222889 for details). It is not practical to revert or change these performance improvements. Thus, the section at https://docs.oracle.com/en/java/javase/12/security/troubleshooting-security.html needs to be updated.

I would suggest that you change the line:

    java -Djava.security.debug=help

to:

    java -Djava.security.debug=help ...

to make it more clear that you need to specify a class or jar file to run (and it could be a simple "Hello World" program). Perhaps you could also add a note indicating that not specifying a class or jar is no longer supported and does not work.