Fix unknown sprite on the readme join button (#510)

utility/downloading_white was a 1.1 sprite. Neither 2.0 nor 2.1 has it,
so drawing the readme for a modded server crashed on_player_created.
utility/downloading exists in both, so no version check is needed.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

exp_scenario/module/gui/readme.lua

@@ -180,7 +180,7 @@ function Elements.join_server.refresh(button, server)
 
     elseif status == "Modded" then
         button.enabled = true
-        button.sprite = "utility/downloading_white"
+        button.sprite = "utility/downloading"
         button.hovered_sprite = "utility/downloading"
 
     else