fix(lint-md-action): use showResult() to output detailed lint errors

[luojiyin1987]

改动

将 showResult() 中的 this.linter.printOverview() 替换为 this.linter.showResult()。

之前只输出一行汇总信息：

Lint total 3 files, 0 warnings 1 errors

现在输出详细的逐文件逐行错误：

/home/user/repo/bad.md
   1:15-1:16             space-round-alphabet          No space between Chinese and alphabet.
   4:1-4:2               no-empty-list                 List content can not be empty.

closes #11