docs: fix screenshot link

Author: corytodd

readme.md

@@ -8,7 +8,7 @@ While working with tables of dense data, my eyes sometimes wander which makes re
 at a glance a challenge. The idea is to provide a black overlay that allows the user
 to click and draw a transparent hole through which interesting data can be viewed.
 
-[demo](screenshots/overlay.png)
+![demo](screenshots/overlay.png)
 
 ## Improvements