Fix documentation of Flow.clipBehavior (#161863)

The documentation described the default value as `Clip.none`, but both
constructors actually default to `Clip.hardEdge`.

(I considered this to be a trivial documentation change, so no issue was
filed.)

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packages/flutter/lib/src/widgets/basic.dart

@@ -5749,7 +5749,7 @@ class Flow extends MultiChildRenderObjectWidget {
 
   /// {@macro flutter.material.Material.clipBehavior}
   ///
-  /// Defaults to [Clip.none].
+  /// Defaults to [Clip.hardEdge].
   final Clip clipBehavior;
 
   @override