chore(docs): use Synth than SynthScope for hashicorp#2755

Modify docs for beginners, in hashicorp#2755.

website/docs/cdktf/test/unit-tests.mdx

@@ -50,19 +50,19 @@ import MyApplicationsAbstraction from "../app"; // Could be a class extending fr
 
 describe("Unit testing using assertions", () => {
   it("should contain a container", () => {
-    expect(
-      Testing.synthScope((scope) => {
-        new MyApplicationsAbstraction(scope, "my-app", {});
-      })
-    ).toHaveResource(Container);
+    const app = Testing.app();
+    const stack = new MyApplicationsAbstraction(app, "my-app", {});
+    const synthesized = Testing.synth(stack);
+
+    expect(synthesized).toHaveResource(Container);
   });
 
   it("should use an ubuntu image", () => {
-    expect(
-      Testing.synthScope((scope) => {
-        new MyApplicationsAbstraction(scope, "my-app", {});
-      })
-    ).toHaveResourceWithProperties(Image, { name: "ubuntu:latest" });
+    const app = Testing.app();
+    const stack = new MyApplicationsAbstraction(app, "my-app", {});
+    const synthesized = Testing.synth(stack);
+
+    expect(synthesized).toHaveResourceWithProperties(Image, { name: "ubuntu:latest" });
   });
 });
 ```