use absolute URL as screenshot source

README.md

@@ -1,5 +1,6 @@
 <a href="http://nicegui.io/#about">
-  <img src="sceenshots/ui-elements-narrow.png" width="200" align="right" alt="Try online!" />
+  <img src="https://raw.githubusercontent.com/zauberzeug/nicegui/main/sceenshots/ui-elements-narrow.png"
+    width="200" align="right" alt="Try online!" />
 </a>
 
 # NiceGUI