update broken link

In 

https://github.com/jlord/sheetsee-maps/blob/master/README.md

#### Source from the [map demo](/demos/demo-map.html):

Link is broken; should be changed to

https://github.com/jlord/sheetsee.js/blob/master/demos/demo-map.html
Merge pull request #2 from tomkeays/master

update broken link