docs: implement ControlValueAccessor in the code snippet (#21905)

`this` should implement `ControlValueAccessor` because
`valueAccessor` has type `ControlValueAccessor | null`
in the `this.ngControl.valueAccessor = this;`

(cherry picked from commit 8726e57)

Author: AlexElin

guides/creating-a-custom-form-field-control.md

@@ -202,7 +202,7 @@ To resolve this, remove the `NG_VALUE_ACCESSOR` provider and instead set the val
     // },
   ],
 })
-export class MyTelInput implements MatFormFieldControl<MyTel> {
+export class MyTelInput implements MatFormFieldControl<MyTel>, ControlValueAccessor {
   constructor(
     ...,
     @Optional() @Self() public ngControl: NgControl,