Update idiomatic constructor example for Point

examples/misc/lib/language_tour/classes/point_alt.dart

@@ -6,12 +6,12 @@
 ///
 // #docregion idiomatic-constructor
 class Point {
-  // Initializer list of variables and values
-  double x = 2.0;
-  double y = 2.0;
+  // Instance variables to hold the coordinates of the point.
+  double x;
+  double y;
 
-  // Generative constructor with initializing formal parameters:
-  Point(this.x, this.y);
+  // Generative constructor with optional positional parameters and default values.
+  Point([this.x = 2.0, this.y = 2.0]);
   // #enddocregion idiomatic-constructor
 
   // #docregion initializer-list