Update AmpTextFormatter.php

Closes #3186193

src/Plugin/Field/FieldFormatter/AmpTextFormatter.php

@@ -14,6 +14,7 @@ use Drupal\Core\Field\FieldItemListInterface;
  *   description = @Translation("Display AMP text."),
  *   field_types = {
  *     "string",
+ *     "string_long",
  *     "text",
  *     "text_long",
  *     "text_with_summary",