chore(docs): fix costs route typo (#114)

Author: avivhalfon

api-reference/costs/property_costs.mdx

@@ -1,6 +1,6 @@
 ---
 title: "Get costs by property"
-api: "GET https://api.traceloop.com/api/v2/costs/by-association-property"
+api: "GET https://api.traceloop.com/v2/costs/by-association-property"
 ---
 
 Query your LLM costs broken down by a specific association property. This helps you understand how costs are distributed across different values of a property (e.g., by user_id, session_id, or any other association property you track).