fixed image not loading in readme

Author: emcramer

README.md

@@ -1,4 +1,4 @@
 # clockplot
 Plotting utility for a "clockplot" that puts groups into a time-ordered heterogeneity visualization.
 
-![A sample clockplot using the built-in lineardemo data](img/demo.PNG)
+<img alt="A sample clockplot using the built-in lineardemo data" src="./img/demo.png" />