Fix screenshot filename case in README

Author: karius-stack

README.md

@@ -3,7 +3,7 @@ Air Notes
 
 Fast, simple flight notes for virtual pilots. The app runs entirely in the browser and stores your data locally via localStorage, so your notes persist between sessions.
 
-![Air Notes screenshot](docs/screenshot.png)
+![Air Notes screenshot](docs/screenshot.PNG)
 
 Contents
 - Overview