Add changeset for attachment readme update (#527)

Author: HeinrichvonStein

.changeset/modern-lobsters-sell.md

@@ -0,0 +1,5 @@
+---
+'@powersync/attachments': patch
+---
+
+Fix README link for StorageAdapter source file (https://github.com/powersync-ja/powersync-js/blob/main/packages/attachments/src/StorageAdapter.ts)