Fix Align widthFactor and heightFactor docs to allow 0 values (flutter#67334)

Author: clocksmith

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

@@ -1835,12 +1835,12 @@ class Align extends SingleChildRenderObjectWidget {
 
   /// If non-null, sets its width to the child's width multiplied by this factor.
   ///
-  /// Can be both greater and less than 1.0 but must be positive.
+  /// Can be both greater and less than 1.0 but must be non-negative.
   final double? widthFactor;
 
   /// If non-null, sets its height to the child's height multiplied by this factor.
   ///
-  /// Can be both greater and less than 1.0 but must be positive.
+  /// Can be both greater and less than 1.0 but must be non-negative.
   final double? heightFactor;
 
   @override