Update payment.md code sample to require ./index.js

The description paragraph mentions:
> It then requires your app at ./index.js to start it up once the SDK is initialized.

This seems to be missing from the code sample.

content/en/docs/demo/services/payment.md

@@ -81,6 +81,8 @@ const sdk = new opentelemetry.NodeSDK({
 });
 
 sdk.start();
+
+require('./index.js');
 ```
 
 You can then use `opentelemetry.js` to start your app. This can be done in the