Update outdated syntax in `JavaScriptBridge` doc

According to `JavaScriptBridge`, thus takes a Callable, not an object/string pair.

doc/classes/JavaScriptObject.xml

@@ -9,7 +9,7 @@
 		[codeblock]
 		extends Node
 
-		var _my_js_callback = JavaScriptBridge.create_callback(self, "myCallback") # This reference must be kept
+		var _my_js_callback = JavaScriptBridge.create_callback(myCallback) # This reference must be kept
 		var console = JavaScriptBridge.get_interface("console")
 
 		func _init():