Fix missing link in alpha release notes

Author: WhyNotHugo

content/blog/2024-05-22-vdirsyncer-2.0.0-alpha0.md

@@ -102,6 +102,8 @@ See the [migration guide] for details on how to update a configuration file.
 There is currently no documentation on setting up a new configuration from
 scratch (this will be addressed at some point on a later release).
 
+[migration guide]: https://git.sr.ht/~whynothugo/vdirsyncer-rs/tree/vdirsyncer/v2.0.0-alpha0/item/docs/content/docs/migration-guide.md
+
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