change the 'src' attribute of <img> pointing to snapshots.png

changed the src from '/images/snapshots.png' to './images/snapshots.png'

Earlier, it didnt load the image when trying to open it in browser
directly from the repo.
Also, "http://jointheconversation.org/git-reference/" does not load the
image, but when the src is changed, the image loads fine.

Author: nvl

index.html

@@ -98,7 +98,7 @@ <h2>How to Think Like Git</h2>
     how they differ or pull content out of them, etc.  
     </p>
 
-    <center><img src="/images/snapshots.png"/></center>
+    <center><img src="./images/snapshots.png"/></center>
 
     <p>
     If you think about Git