fix(prism): use correct type to pass test

[liby]

https://github.com/hexojs/hexo-util/blob/master/lib/prism.ts#LL57-L65C2

interface Options {
  caption?: string;
  firstLine?: number;
  isPreprocess?: boolean;
  lang?: string;
  lineNumber?: boolean;
  mark?: string;
  tab?: string;
}

[coveralls]

Coverage: 95.527%. Remained the same when pulling 782f045 on liby:master into f53c77d on hexojs:master.

[yoshinorin]

Thank you ❤️