fix RaduisExtension name (#556)

Change RaduisExtension name to RadiusExtension

@@ -138,7 +138,7 @@ extension BorderRaduisExtension on BorderRadius {
       );
 }
 
-extension RaduisExtension on Radius {
+extension RadiusExtension on Radius {
   /// Creates adapt Radius using r [SizeExtension].
   Radius get r => Radius.elliptical(x.r, y.r);