Repro

{
  "rules": {
    "@typescript-eslint/prefer-readonly-parameter-types": "error"
  }
}

interface CustomArrayType extends ReadonlyArray<string> {
  readonly prop: string;
}
function custom1(arg: CustomArrayType) {}

Expected Result
No error

Actual Result
Parameter should be a read only type

Additional Info
This is the example within https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/prefer-readonly-parameter-types.md

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