[Cloudflare One] Fix Replace VPN sidebar link to point to learning path (#29128)

The sidebar external_link was pointing to the onboarding guide instead
of the learning path. The card on the same page was already corrected
in PR #28903 but the sidebar link was missed.

Author: codyanthony850

src/content/docs/cloudflare-one/implementation-guides/replace-vpn.mdx

@@ -1,7 +1,7 @@
 ---
 pcx_content_type: navigation
 title: Replace your VPN
-external_link: /cloudflare-one/setup/replace-vpn/
+external_link: /learning-paths/replace-vpn/concepts/
 sidebar:
   order: 2
 ---