fix Found a Bug Go Here url

Hey, great work on hextris!

The current url for **Found a bug? Go here** points to http://hextris.github.io/hextris/url (which is not a valid page)

Changed it so it goes to https://github.com/Hextris/hextris/issues

(also added a new line at the end of the file. please let me know if you want me to take it out)