Fix warning

Signed-off-by: Christopher Ng <chrng8@gmail.com>

apps/dav/lib/UserMigration/ContactsMigrator.php

@@ -168,7 +168,7 @@ class ContactsMigrator implements IMigrator {
 		}
 
 		$existingAddressBookUris = array_map(
-			fn (array $addressBookInfo) => $addressBookInfo['uri'],
+			fn (array $addressBookInfo): string => $addressBookInfo['uri'],
 			$this->cardDavBackend->getAddressBooksForUser($principalUri),
 		);