Fix the plugins in the demo did not load successfully

[packbase]

I tried running run-demo.sh and found that the results were not correct: WelcomePlugin and HelloPlugin could be loaded, but the extensions didn't work.

The log message:

Found 1 extensions for extension point 'org.pf4j.demo.api.Greeting'
>>> Whazzup

WelcomePlugin.start()
HelloPlugin.start()

HelloPlugin.stop()
WelcomePlugin.stop()

I found it was due to a modification of a commit: 254c9cc

The log after the repair is:

Found 3 extensions for extension point 'org.pf4j.demo.api.Greeting'
>>> Whazzup
>>> Welcome
>>> Hello

WelcomePlugin.start()
HelloPlugin.start()

HelloPlugin.stop()
WelcomePlugin.stop()

Thanks.

[decebals]

Thanks!