Resolve #3412857 "Error message there"

Closes #3412857

src/Plugin/Field/FieldType/SerialItem.php

@@ -17,7 +17,7 @@ use Drupal\Core\TypedData\TranslatableInterface;
  *   id = "serial",
  *   label = @Translation("Serial"),
  *   description = @Translation("Auto increment serial field type."),
- *   category = "Number",
+ *   category = "number",
  *   default_widget = "serial_default_widget",
  *   default_formatter = "serial_default_formatter"
  * )