fixing some errors from nextjs-source-map-updates branch

Author: Shannon Anahata

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

@@ -30,7 +30,7 @@ module.exports = withSentryConfig(
 );
 ```
 
-#### 2. Source Map Options
+#### 2. [Optional] Customize with 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, // Disable source maps (default = true)
       assets: ["**/*.js", "**/*.js.map"], // Specify which files to upload
       ignore: ["**/node_modules/**"], // Files to exclude
       deleteSourcemapsAfterUpload: true, // Security: delete after upload