Wrong path for Default Sitemap

[navjotjsingh]

The default path in the documentation/configuration file should be

path: /sitemap.xml

Without using any /, the URL that generates in the header is http://example.comsitemap.xml.

[uiolee]

in _config.yml, root should be set as '/'

related code:

https://github.com/hexojs/hexo-generator-sitemap/blob/master/lib/rel.js#L10

related doc: https://github.com/hexojs/hexo-util#url_forpath-option