fix: update default close icon

problem: nf-midi-close was removed from nerdfonts
solution: replace it with nf-md-close.

closes #148

lua/cokeline/config.lua

@@ -84,7 +84,7 @@ local defaults = {
       text = " ",
     },
     {
-      text = "",
+      text = "󰅖",
       on_click = function(_, _, _, _, buffer)
         buffer:delete()
       end,