Update src/content/docs/en/guides/upgrade-to/v6.mdx

Co-authored-by: Sarah Rainsberger <[email protected]>

src/content/docs/en/guides/upgrade-to/v6.mdx

@@ -137,7 +137,7 @@ Notably, many `string()` formats have been deprecated (e.g. `z.string().email()`
   }
   ```
 
-Additionally, if you use `.default()` with transforms, you may need to update your schemas. In Zod 4, default values must match the output type (after transforms), not the input type. The default value short-circuits parsing when the input is `undefined`:
+Additionally, if you use [`.default()` with transforms](https://zod.dev/v4/changelog#default-updates), you may need to update your schemas. In Zod 4, default values must match the output type (after transforms), not the input type. The default value short-circuits parsing when the input is `undefined`:
 
 ```ts title="src/content.config.ts" del={5-6} ins={7-8}
 import { z } from 'astro/zod';