Update security-headers doc (vercel#41312)

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## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

Just fixing an error in the documentation where the text doesn't match
what the code snippet is doing.

Co-authored-by: JJ Kasper <jj@jjsweb.site>

docs/advanced-features/security-headers.md

@@ -130,9 +130,9 @@ const ContentSecurityPolicy = `
 
 When a directive uses a keyword such as `self`, wrap it in single quotes `''`.
 
-In the header's value, replace the new line with an empty string.
+In the header's value, replace the new line with a space.
 
-```jsx
+```js
 {
   key: 'Content-Security-Policy',
   value: ContentSecurityPolicy.replace(/\s{2,}/g, ' ').trim()