docs(forms): make onSubmit method async for form submission

Change onSubmit method to be asynchronous since we are using await inside it

Author: zakaria-bali

adev/src/content/guide/forms/signals/models.md

@@ -117,7 +117,7 @@ You can access form values in two ways: directly from the model signal, or throu
 Access the model signal when you need the complete form data, such as during form submission:
 
 ```ts
-onSubmit() {
+async onSubmit() {
   const formData = this.loginModel();
   console.log(formData.email, formData.password);