Fix documentation typo in basic.dart (#143887)

Fixes a small documentation typo.

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

@@ -5203,7 +5203,7 @@ class Flexible extends ParentDataWidget<FlexParentData> {
   /// The flex factor to use for this child.
   ///
   /// If null or zero, the child is inflexible and determines its own size. If
-  /// non-zero, the amount of space the child's can occupy in the main axis is
+  /// non-zero, the amount of space the child can occupy in the main axis is
   /// determined by dividing the free space (after placing the inflexible
   /// children) according to the flex factors of the flexible children.
   final int flex;