Incorrect `formatString` return type

[cumpstey]

The formatString method is defined as:

    formatString<T extends Formatted>(
      str: string,
      ...values: Array<T | FormatObject<T>>
    ): Array<string | T> | string;

The return type should be just string. As far as I can tell, this doesn't require any changes to the actual method - just the .d.ts.