fix: use the new screenshot URL

Author: MightyCreak

data/io.github.mightycreak.Diffuse.appdata.xml.in

@@ -19,7 +19,7 @@
   <screenshots>
     <screenshot type="default">
       <caption>Main window: diff between two files</caption>
-      <image>https://mightycreak.github.io/diffuse/images/screenshot_v0.7.0.png</image>
+      <image>https://mightycreak.github.io/diffuse/docs/images/screenshot_v0.7.0.png</image>
     </screenshot>
   </screenshots>