Fix wrong Icon name in minetest.desktop

misc/minetest.desktop

@@ -3,7 +3,7 @@ Name=Minetest
 GenericName=Minetest
 Comment=Multiplayer infinite-world block sandbox
 Exec=minetest
-Icon=minetest-icon
+Icon=minetest
 Terminal=false
 Type=Application
 Categories=Game;