changed to lowercase PNG for header image in docs

Author: Wes Williams

Documentation/index.html

@@ -19,7 +19,7 @@
 		<div id="TopColorbar"> </div>
 		<div id="TitleBar">
 			<div>
-				<a href="http://pdn.pearson.com" target="_blank" title="Home"><img src="assets/pdn-header-sm.PNG" /></a>
+				<a href="http://pdn.pearson.com" target="_blank" title="Home"><img src="assets/pdn-header-sm.png" /></a>
 				<a href="http://pdn.pearson.com" target="_blank">pdn.pearson.com</a>
 			</div>
 		</div>