docs: fix broken link on middleware setup (#13253)

Author: coderrshyam

docs/pages/getting-started/migrate-to-better-auth.mdx

@@ -155,7 +155,7 @@ export const protectedAction = async () => {
 
 ### Middleware
 
-To protect routes with middleware, refer to the [Next.js middleware guide](/docs/integrations/next#middleware).
+To protect routes with middleware, refer to the [Next.js middleware guide](https://www.better-auth.com/docs/integrations/next#middleware).
 
 </Steps>