fixed URLs in postgis-101 slides by making them relative paths [ref #1] 


I fixed the URLs that was in #1 by making them relative, which is nice for github and for anyone who wants to fork the repo. 
see https://help.github.com/articles/relative-links-in-readmes

Only as I was typing this PR up that making the links relative would be problematic for biggie, because it results in broken images for biggie; which is what you (@smathermather) used for slides.  

Should we just continue with the absolute paths for the images then?