fix: couldn't disable buffering_indicator due to incorrect ID (#809)

The element id didn't match the id in the constructors table or the
description in the config. Consequently it was possible to disable the
indicator at startup, but once it was instantiated it wasn't possible to
remove it again.

Fixes #808

src/uosc/elements/BufferingIndicator.lua

@@ -5,7 +5,7 @@ local BufferingIndicator = class(Element)
 
 function BufferingIndicator:new() return Class.new(self) --[[@as BufferingIndicator]] end
 function BufferingIndicator:init()
-	Element.init(self, 'buffer_indicator', {ignores_curtain = true, render_order = 2})
+	Element.init(self, 'buffering_indicator', {ignores_curtain = true, render_order = 2})
 	self.enabled = false
 	self:decide_enabled()
 end