docs-bug(MatFormField): inconsistency between Custom form field guide and live example for `errorState`

[AlexElin]

Documentation Feedback

In the Custom form field guide the errorState is just set to false.
But in the related live example it has another implementation:

get errorState(): boolean {
    return this.parts.invalid && this.parts.dirty;
}

There's need to eliminate inconsistency between the guide and the live example

Affected documentation page: https://material.angular.io/guide/creating-a-custom-form-field-control#errorstate

[annieyw]

Confirmed that the guide should be updated to reflect the live example.

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