Fixed screenshot

Signed-off-by: Blagovest Petrov <blagovest@petrovs.info>

org.petrovs_info.tangraplay.appdata.xml

@@ -15,7 +15,7 @@
   </releases>
   <screenshots>
     <screenshot type="default">
-      <image type="source">https://tangraplay.petrovs.info/screenshot.png</image>
+      <image type="source">https://tangraplay.petrovs.info/images/tangraplay.png</image>
     </screenshot>
   </screenshots>
   <categories>