Grammar fix: "it's" -> "its" (#48913)

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

@@ -4367,7 +4367,7 @@ class Column extends Flex {
 /// See also:
 ///
 ///  * [Expanded], which forces the child to expand to fill the available space.
-///  * [Spacer], a widget that takes up space proportional to it's flex value.
+///  * [Spacer], a widget that takes up space proportional to its flex value.
 ///  * The [catalog of layout widgets](https://flutter.dev/widgets/layout/).
 class Flexible extends ParentDataWidget<FlexParentData> {
   /// Creates a widget that controls how a child of a [Row], [Column], or [Flex]