correcting 'sitemap' show flag for 2.7

The sitemap flag did not show up in the page edit overlay when setting these config options:

sitemap:
  show_root: true
  show_flag: true

Searching through the more recent source code, I realized that you were referring to :show_flag instead of 'show_flag'. I tested this using my fork and it works as it should. Please pull this change so I can edit my site's sitemap. 

Thanks!