fix syntax error in comment pseudocode (#162453)

Seems this pseudocode has been overlooked during null safety migration.

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packages/flutter/lib/src/rendering/object.dart

@@ -4051,7 +4051,7 @@ abstract class RenderObject with DiagnosticableTreeMixin implements HitTestTarge
   // (with the signature being whatever passes for coordinates for this
   // particular class):
   //
-  // bool hitTest(HitTestResult result, { Offset position }) {
+  // bool hitTest(HitTestResult result, { required Offset position }) {
   //   // If the given position is not inside this node, then return false.
   //   // Otherwise:
   //   // For each child that intersects the position, in z-order starting from