doc: fix javadoc in MetamodelPropertyField.java

[zielint0]

Code inspection

• Declaration has problems in Javadoc references inspection

Symbol 'simpleName' is inacessible from here

/**
 * Tells that a field is a property of the metamodel
 * For instance {@link CtClassImpl#simpleName} is the property name of {@link CtNamedElement}
 *
 */
@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.FIELD })
public @interface MetamodelPropertyField {
	CtRole[] role();
}

Maybe a correct solution is reference to:
{@link spoon.support.reflect.declaration.CtNamedElementImpl#getSimpleName}
?

@pvojtechovsky
Could you take a look on WHOLE comment? ;-)