docs: update Cubic constructor doc. (#103555)

packages/flutter/lib/src/animation/curves.dart

@@ -302,7 +302,7 @@ class Cubic extends Curve {
   /// Rather than creating a new instance, consider using one of the common
   /// cubic curves in [Curves].
   ///
-  /// The [a] (x1), [b](x2), [c](y1), and [d](y2) arguments must not be null.
+  /// The [a] (x1), [b] (y1), [c] (x2) and [d] (y2) arguments must not be null.
   const Cubic(this.a, this.b, this.c, this.d)
     : assert(a != null),
       assert(b != null),