docs: Remove explicit reference to string transform in attr()

Author: muziejus

docs/ember-data/models.md

@@ -23,7 +23,7 @@ import Model, { attr } from '@ember-data/model';
 import CustomType from '../transforms/custom-transform';
 
 export default class User extends Model {
-  @attr('string')
+  @attr()
   declare name?:  string;
 
   @attr('number')
@@ -47,7 +47,7 @@ One way to make this safer is to supply a default value using the `defaultValue`
 import Model, { attr } from '@ember-data/model';
 
 export default class User extends Model {
-  @attr('string')
+  @attr()
   declare name?:  string;
 
   @attr('number', { defaultValue: 13 })