Add concurrency add-on pricing to billing page

Author: beran-t

docs/billing.mdx

@@ -18,7 +18,7 @@ E2B uses [usage-based pricing](#usage-based-pricing) - you pay only for what you
 | **Concurrent sandboxes** | 20 | 100 - 1,100 | 1,100+ |
 
 <Note>
-Pro plan includes 100 concurrent sandboxes. Higher concurrency up to 1,100 is available as a purchasable [add-on](https://e2b.dev/dashboard?tab=billing).
+Pro plan includes 100 concurrent sandboxes. Higher concurrency up to 1,100 is available as an [add-on](https://e2b.dev/dashboard?tab=billing) for $500/month.
 </Note>
 
 Plans have different [API rate limits](/docs/sandbox/rate-limits).