Update onXExample_pre.jpage

[jevanlingen]

Update onX example, to make the JDK 8+ onConstructor syntax more clear.

Notice, the written documentation is already on point:

On javac7, to use any of the 3 onX features, you must wrap the annotations to be applied to the constructor / method / parameter in @__(@AnnotationGoesHere). On javac8 and up, you add an underscore after onMethod, onParam, or onConstructor.

[jevanlingen]

The documentation also mentioned:

Possibly java 9 will offer (much) better ways of supporting this feature.

I don't know if java 9 did support this better, but at least that line should be updated (or removed) as well.