fix(readme): add await to prompt.stream example (copilot-extensions#112)

Adds a small change to the `README.md` file. The change updates the
example code to use `await` with the `prompt.stream` method for proper
asynchronous handling.

Author: tspascoal

README.md

@@ -461,7 +461,7 @@ Works the same way as `prompt()`, but resolves with a `stream` key instead of a
 ```js
 import { prompt } from "@copilot-extensions/preview-sdk";
 
-const { requestId, stream } = prompt.stream("What is the capital of France?", {
+const { requestId, stream } = await prompt.stream("What is the capital of France?", {
   token: process.env.TOKEN,
 });