Update: Performance JS instructions for CDN and without CDN (#1858)

Updated code example to have options for CDN and without CDN.

Author: MimiDumpling

src/collections/_documentation/performance-monitoring/configuration/javascript.md

@@ -44,9 +44,21 @@ The `Tracing` integration is specific to `@sentry/browser` and does not work wit
 The `Tracing` integration resides in the `@sentry/apm` package. You can add it to your `Sentry.init` call:
 
 ```javascript
+// Without CDN
+
 import * as Sentry from '@sentry/browser';
 import { Integrations as ApmIntegrations } from '@sentry/apm';
 
+Sentry.init({
+  dsn: '___PUBLIC_DSN___',
+  integrations: [
+    new ApmIntegrations.Tracing(),
+  ],
+  tracesSampleRate: 0.25,
+});
+
+// With CDN
+
 Sentry.init({
   dsn: '___PUBLIC_DSN___',
   integrations: [