docs(toolkit-lib): await cdk.fromAssemblyBuilder in readme (#258)

In subsequent examples we type `cx` as `ICloudAssemblySource`, so it
makes more sense for our examples to show `cx` as an
`ICloudAssemblySource` and not `Promise<ICloudAssemblySource>`.

---
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under the terms of the Apache-2.0 license

Author: kaizencc

packages/@aws-cdk/toolkit-lib/README.md

@@ -61,7 +61,7 @@ import * as core from 'aws-cdk-lib/core';
 
 declare const cdk: Toolkit;
 
-const cx = cdk.fromAssemblyBuilder(async () => {
+const cx = await cdk.fromAssemblyBuilder(async () => {
   const app = new core.App();
 
   // Define your stacks here
@@ -321,7 +321,7 @@ Alternatively a inline `AssemblyBuilder` function can be used to build a CDK app
 ```ts
 declare const cdk: Toolkit;
 
-const cx = cdk.fromAssemblyBuilder(async () => {
+const cx = await cdk.fromAssemblyBuilder(async () => {
   const app = new core.App();
 
   // Define your stacks here