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

Co-authored-by: Sarah Rainsberger <5098874+sarah11918@users.noreply.github.com>

Author: florian-lefebvre

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

@@ -551,7 +551,7 @@ namespace NodeJS {
 
 If you need more control over environment variables in Astro, we recommend you use `astro:env`.
 
-<ReadMore>Learn more about [environment variables](/en/guides/environment-variables/).</ReadMore>
+<ReadMore>Learn more about [environment variables](/en/guides/environment-variables/) in Astro, including `astro:env`.</ReadMore>
 
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