Add screenshot and update screenshot URL in metadata

Hopefully the metadata update allows Flatpak-builder to stop complaining.

Author: bryce-carson

org.messerlab.slimgui.appdata.xml

@@ -13,7 +13,7 @@
   </description>
   <screenshots>
     <screenshot type="default">
-        <image>https://raw.githubusercontent.com/bryce-carson/SLiM/ab1a982d6f6238d1702cd6a146b4b90019a566d4/org.messerlab.slimgui.screenshot.png</image>
+        <image>https://raw.githubusercontent.com/bryce-carson/SLiM/master/org.messerlab.slimgui.screenshot.png</image>
     </screenshot>
   </screenshots>
   <!-- LINKS to display in "Software Center" applications-->

org.messerlab.slimgui.metainfo.xml

@@ -17,7 +17,7 @@
   <launchable type="desktop-id">org.messerlab.slimgui.desktop</launchable>
   <screenshots>
       <screenshot type="default">
-          <image>https://raw.githubusercontent.com/bryce-carson/SLiM/ab1a982d6f6238d1702cd6a146b4b90019a566d4/org.messerlab.slimgui.screenshot.png</image>
+          <image>https://raw.githubusercontent.com/bryce-carson/SLiM/master/org.messerlab.slimgui.screenshot.png</image>
       </screenshot>
   </screenshots>
   <!-- LINKS to display in "Software Center" applications-->