docs(js): Update Bun quick start guide: users need to preload instrumentation file (#15575)

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## DESCRIBE YOUR PR
Updated Bun's Quick Start guide:
- users need to `--preload` their instrumentation file to make sure Bun
loads Sentry before other modules

Closes: #15539 

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## PRE-MERGE CHECKLIST

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## EXTRA RESOURCES

- [Sentry Docs contributor guide](https://docs.sentry.io/contributing/)

---------

Co-authored-by: Sigrid <[email protected]>

Author: inventarSarah

docs/platforms/javascript/guides/bun/index.mdx

@@ -34,7 +34,7 @@ bun add @sentry/bun
 ### Initialize the Sentry SDK
 
 Sentry should be initialized as early in your app as possible.
-To import and initialize Sentry, create a file named `instrument.js` in the root directory of your project and add the following code:
+Create a file named `instrument.js` in the root directory of your project and add the following code:
 
 ```javascript {filename: instrument.js}
 import * as Sentry from "@sentry/bun";
@@ -65,16 +65,11 @@ Sentry.init({
 
 ### Apply Instrumentation to Your App
 
-Import `instrument.js` before any other modules to make sure Sentry initializes early. If you initialize later, auto-instrumentation and modules like tracing may not work.
+To make sure that Sentry initializes before any other modules, you need to preload your instrumentation file using the [`--preload`](https://bun.com/docs/runtime#param-preload) flag.
+Start your app using:
 
-```javascript {filename: app.js}
-// Import this first!
-import "./instrument";
-
-// Now import other modules
-import http from "http";
-
-// Your application code goes here
+```bash
+bun --preload ./instrument.js app.js
 ```
 
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