fixed image link in 'fork me on github' snippet

more details in issue #10

this fixes the broken image link, keeping the use of single quotes, but changing the link to use https and removing the z-index setting, just going by the snippets at the github ribbons page @ https://github.com/blog/273-github-ribbons
Merge pull request #11 from jamesmanning/patch-1

fixed image link in 'fork me on github' snippet