Update postmaster.mdx (cloudflare#23518)

Fix the link to https://developers.cloudflare.com/changelog/2025-06-30-mail-authentication/

Author: vitch

src/content/docs/email-routing/postmaster.mdx

@@ -53,7 +53,7 @@ It is recommended that all senders implement the DMARC protocol in order to succ
 
 ### Mail authentication requirement
 
-Cloudflare requires emails to [pass some form of authentication](/changelog/email-routing/2025-06-30-mail-authentication/), either pass SPF verification or be correctly DKIM-signed to forward them. Having DMARC configured will also have a positive impact and is recommended.
+Cloudflare requires emails to [pass some form of authentication](/changelog/2025-06-30-mail-authentication/), either pass SPF verification or be correctly DKIM-signed to forward them. Having DMARC configured will also have a positive impact and is recommended.
 
 ### IPv6 support