Update entity_browser.api.php

I just noticed that the entity_browser.api.php needs updating

Some functions are using variables that do not exist in the context i.e.

 function hook_entity_browser_widget_validation_info_alter(array &$validation_plugins) {
   $field_displays['not_null']['label'] = t('Not null fabulous validator');

Should be

 function hook_entity_browser_widget_validation_info_alter(array &$validation_plugins) {
   $validation_plugins['not_null']['label'] = t('Not null fabulous validator');

I am uploading a patch with these fixes and adding a bit more of information.