Use JSLink to edit layout of Document library webpart

This is the first time I try to develop something using JSLink so I struggle a bit ;-)

I have a standard document library web-part on my page with a few fields. For my test I want to change the custom field "LinkTest". My ultimate goal is to change the icon of a certain content-type. But I cut this challenge in pieces. The first part is to change the "LinkTest" field.

I have this piece of code:

(function() {
  
var mjh = mjh || {};
mjh.displayMethod = function (ctx) {
  return '<div>Hello World</div>';
}

var mjhOverrides = {};
  mjhOverrides.Templates = {};
  mjhOverrides.Templates.Fields = {  
    'LinkTest': {
      'DisplayForm': mjh.displayMethod
    }
  };

  SPClientTemplates.TemplateManager.RegisterTemplateOverrides(mjhOverrides );
})();

When I debug I see that the code is executed without (visual) errors.

But ..... nothing happens.

What is wrong in my approach?

Thanks, Mike