Pass filename as `name` instead of passing full path as `dir`

`mtime` method [has added](https://github.com/jekyll/jekyll/blob/74f0f27d18571d41eb6988a2f1e72c19c22f4c73/lib/jekyll/static_file.rb#L40) to `StaticFile`. It calls `File.stat()` and it occurs error:

```
/path/to/jekyll/lib/jekyll/static_file.rb:40:in `stat': Not a directory @ rb_file_s_stat - /path/to/generated/alias/index.html/ (Errrno::ENOTDIR)
```