Fixes gist URL API

Adds a missing '/' in the URL used to fetch a gist.

For example, here's the link in API.md which is currently broken:
http://geojson.io/#id=gist:tmcw/e9a29ad54dbaa83dee08&map=8/39.198/-76.981

Previously was trying to fetch URLs like: https://api.github.com/gistsabcde - which would 404.
Proper URL is like: https://api.github.com/gists/abcde