Set indentation to 3 for walkthrough project

For eng/ide/als-publish#12

Author: AnthonyLeonardoGracio

integration/vscode/ada/src/walkthrough.ts

@@ -35,9 +35,9 @@ end Hello_World;
             `with Ada.Text_IO;
 
 procedure Hello_World is
-    Msg : constant String := "Hello World!";
+   Msg : constant String := "Hello World!";
 begin
-    Ada.Text_IO.Put_Line (Msg);
+   Ada.Text_IO.Put_Line (Msg);
 end Hello_World;
 `,
         );