docs: fix PostHog API Key link (#14433)

Update PostHog API Key link in page.mdx

www/apps/resources/app/infrastructure-modules/analytics/posthog/page.mdx

@@ -30,7 +30,7 @@ The Analytics Module and its providers are available starting [Medusa v2.8.3](ht
     },
     {
       text: "PostHog API Key",
-      link: "https://posthog.com/docs/getting-started/api-key",
+      link: "https://posthog.com/docs/api",
     },
   ]}
 />
@@ -227,4 +227,4 @@ You'll now track the order placement event whenever an order is placed in your M
 
 ## Additional Resources
 
-- [How to Use the Analytics Module](/references/analytics/service)
+- [How to Use the Analytics Module](/references/analytics/service)