Fix Flex class docs by replacing 'vertical space' with 'space on its main...

Fix Flex class docs by replacing 'vertical space' with 'space on its main axis' so that the language fits both column and row. (#29057)

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

@@ -3568,8 +3568,8 @@ class PositionedDirectional extends StatelessWidget {
 /// horizontal) or [Column] (if it's vertical) instead, because that will be less
 /// verbose.
 ///
-/// To cause a child to expand to fill the available vertical space, wrap the
-/// child in an [Expanded] widget.
+/// To cause a child to expand to fill the available space in the [direction]
+/// of this widget's main axis, wrap the child in an [Expanded] widget.
 ///
 /// The [Flex] widget does not scroll (and in general it is considered an error
 /// to have more children in a [Flex] than will fit in the available room). If