Fix property name for `max_line_length` in README

According to [editorconfig wiki](https://github.com/editorconfig/editorconfig/wiki/EditorConfig-Properties#max_line_length) and [plugin code](https://github.com/editorconfig/editorconfig-vim/blob/master/plugin/editorconfig.vim#L543), `max_line_length` is the correct propery name, not `max_line_width`.