fix: fix typo in $_sizes map (@W-11847644) (#5151)

Author: nolanlawson

ui/utilities/sizing/_index.scss

@@ -5,7 +5,7 @@
 /// @type map
 /// @private
 $_sizes: (
-  'xxx-small,': $size-xxx-small,
+  'xxx-small': $size-xxx-small,
   'xx-small': $size-xx-small,
   'x-small': $size-x-small,
   'small': $size-small,