Update shape.cs (dotnet#2862)

* Update shape.cs

Math.Pow(Math.PI * Radius, 2) should be Math.Pi * Radius * Radius since Area = pi * Radius^2 = pi * Radius * Radius

* Update shape.cs

Math.Pi => Math.PI

* Update shape.cs

* Update shape.cs

* Update shape.cs

Author: IvenBach

samples/snippets/csharp/tutorials/inheritance/shape.cs

@@ -64,7 +64,7 @@ public Circle(double radius)
       Radius = radius;
    } 
 
-   public override double Area => Math.Round(Math.Pow(Math.PI * Radius, 2), 2); 
+   public override double Area => Math.Round(Math.PI * Math.Pow(Radius, 2), 2); 
 
    public override double Perimeter => Math.Round(Math.PI * 2 * Radius, 2); 
 
@@ -112,6 +112,6 @@ public static void Main()
 //            Is Square: False, Diagonal: 15.62
 //         Square: area, 25; perimeter, 20
 //            Diagonal: 7.07
-//         Circle: area, 88.83; perimeter, 18.85
+//         Circle: area, 28.27; perimeter, 18.85
 // </Snippet3>
-} // Namespace definition.
+} // Namespace definition.