fixing some errors from nextjs-source-map-updates branch (#14467)

## DESCRIBE YOUR PR
- Fix enabled/disabled typo
- Make clear that default: true is the default
- Make clearer that source maps options are optional for set-up
- 
## IS YOUR CHANGE URGENT?  

Help us prioritize incoming PRs by letting us know when the change needs
to go live.
- [ ] Urgent deadline (GA date, etc.): <!-- ENTER DATE HERE -->
- [ ] Other deadline: <!-- ENTER DATE HERE -->
- [x] None: Not urgent, can wait up to 1 week+

## SLA

- Teamwork makes the dream work, so please add a reviewer to your PRs.
- Please give the docs team up to 1 week to review your PR unless you've
added an urgent due date to it.
Thanks in advance for your help!

## PRE-MERGE CHECKLIST

*Make sure you've checked the following before merging your changes:*

- [ ] Checked Vercel preview for correctness, including links
- [ ] PR was reviewed and approved by any necessary SMEs (subject matter
experts)
- [ ] PR was reviewed and approved by a member of the [Sentry docs
team](https://github.com/orgs/getsentry/teams/docs)

---------

Co-authored-by: Shannon Anahata <[email protected]>
Co-authored-by: Charly Gomez <[email protected]>

Author: 3 people

platform-includes/sourcemaps/overview/javascript.nextjs.mdx

@@ -30,7 +30,7 @@ module.exports = withSentryConfig(
 );
 ```
 
-#### 2. Source Map Options
+#### 2. [Optional] Customize Source Map Options
 
 Configure source map behavior using the [source maps options](/platforms/javascript/guides/nextjs/configuration/build/#source-maps-options) in your Next.js config:
 
@@ -46,7 +46,7 @@ module.exports = withSentryConfig(
     project: "___PROJECT_SLUG___",
     authToken: process.env.SENTRY_AUTH_TOKEN,
     sourcemaps: {
-      disable: false, // Enable source maps (default: false)
+      disable: false, // Source maps are enabled by default
       assets: ["**/*.js", "**/*.js.map"], // Specify which files to upload
       ignore: ["**/node_modules/**"], // Files to exclude
       deleteSourcemapsAfterUpload: true, // Security: delete after upload