correct information about record identification

The description claimed that `.new_record?` was used in `form_for` to derive the action and button text.
But in the code `.persisted?` is used for that purpose.

[ci skip]

guides/source/form_helpers.md

@@ -317,7 +317,7 @@ form_with(model: @article, url: article_path(@article), method: "patch")
 form_with(model: @article)
 ```
 
-Notice how the short-style `form_with` invocation is conveniently the same, regardless of the record being new or existing. Record identification is smart enough to figure out if the record is new by asking `record.new_record?`. It also selects the correct path to submit to, and the name based on the class of the object.
+Notice how the short-style `form_with` invocation is conveniently the same, regardless of the record being new or existing. Record identification is smart enough to figure out if the record is new by asking `record.persisted?`. It also selects the correct path to submit to, and the name based on the class of the object.
 
 WARNING: When you're using STI (single-table inheritance) with your models, you can't rely on record identification on a subclass if only their parent class is declared a resource. You will have to specify `:url`, and `:scope` (the model name) explicitly.