fix: Ktor Subscriptions wrong url #1863 (#1865)

### 📝 Description
Replaced Spring subscriptions URL with Ktor subscriptions.

### 🔗 Related Issues
#1863

Author: charlee-dev

website/docs/server/server-subscriptions.md

@@ -10,4 +10,4 @@ specific details on how to read incoming messages from the WebSocket session as
 If you are using one of the official server implementations for GraphQL Kotlin, it will have subscription handling setup for you.
 
 -   See `graphql-kotlin-spring-server` [subscriptions](spring-server/spring-subscriptions.md)
--   See `graphql-kotlin-ktor-server` [subscriptions](spring-server/spring-subscriptions.md)
+-   See `graphql-kotlin-ktor-server` [subscriptions](ktor-server/ktor-subscriptions.md)