docs(form-field): use value getter when returning form data (#18609)

(cherry picked from commit 33f977c)

Author: tobiasschweizer

src/components-examples/material-experimental/mdc-form-field/mdc-form-field-custom-control/form-field-custom-control-example.ts

@@ -147,7 +147,7 @@ export class MyTelInput implements ControlValueAccessor, MatFormFieldControl<MyT
   }
 
   _handleInput(): void {
-    this.onChange(this.parts.value);
+    this.onChange(this.value);
   }
 
   static ngAcceptInputType_disabled: boolean | string | null | undefined;

src/components-examples/material/form-field/form-field-custom-control/form-field-custom-control-example.ts

@@ -147,7 +147,7 @@ export class MyTelInput implements ControlValueAccessor, MatFormFieldControl<MyT
   }
 
   _handleInput(): void {
-    this.onChange(this.parts.value);
+    this.onChange(this.value);
   }
 
   static ngAcceptInputType_disabled: boolean | string | null | undefined;