fix OpenRouter API URL instruction in documentation (cloudflare#25362)

Author: fernandez14

src/content/docs/ai-gateway/usage/providers/openrouter.mdx

@@ -13,7 +13,7 @@ https://gateway.ai.cloudflare.com/v1/{account_id}/{gateway_id}/openrouter
 
 ## URL structure
 
-When making requests to [OpenRouter](https://openrouter.ai/), replace `https://openrouter.ai/api/v1/chat/completions` in the URL you are currently using with `https://gateway.ai.cloudflare.com/v1/{account_id}/{gateway_id}/openrouter`.
+When making requests to [OpenRouter](https://openrouter.ai/), replace `https://openrouter.ai/api/v1/chat/completions` in the URL you are currently using with `https://gateway.ai.cloudflare.com/v1/{account_id}/{gateway_id}/openrouter/chat/completions`.
 
 ## Prerequisites