Update README.md: fixed docs of FlxUICheckBox

[giuppe]

This is about the docs of FlxUICheckBox in the readme file:

• the descriptions for the arguments check_src and box_src were swapped;
• the click event was reported as click_checkbox but it really is click_check_box:

  flixel-ui/flixel/addons/ui/FlxUICheckBox.hx

  Line 41 in 2756177

  public static inline var CLICK_EVENT:String = "click_check_box";

• the params of the click_check_box event was described as a single string in the form checked:true while it really is an object of type {name:String, value:Bool} (an FlxUI.NamedBool):

  flixel-ui/flixel/addons/ui/FlxUICheckBox.hx

  Line 57 in 2756177

  var nb:NamedBool = {name: "checked", value: false};

[Geokureli]

thanks!