Resolve Typescript validation check failed (#7639)

The hander definition previously didn't include a type so the "npx ampx sandbox" compilation would fail and say:
amplify/functions/say-hello/handler.ts(1,31): error TS7006: Parameter 'event' implicitly has an 'any' type.
TypeScript validation check failed.
Resolution: Fix the syntax and type errors in your backend definition.

Author: jpangburn

src/pages/[platform]/build-a-backend/functions/set-up-function/index.mdx

@@ -53,7 +53,9 @@ export const sayHello = defineFunction({
 Next, create the corresponding handler file at `amplify/functions/say-hello/handler.ts`. This is where your function code will go.
 
 ```ts title="amplify/functions/say-hello/handler.ts"
-export const handler = async (event) => {
+import { Handler } from 'aws-lambda';
+
+export const handler: Handler = async (event, context) => {
   // your function code goes here
   return 'Hello, World!';
 };