Fix perspectives project link
The perspectives project seems to be dead :/. We can at least link to the
wayback machine.
@@ -111,7 +111,7 @@ avoiding a single trust-root since each server is free to pick which notary
servers they trust and can corroborate the keys returned by a given notary
server by querying other servers.
-.. _Perspectives Project: http://perspectives-project.org/
+.. _Perspectives Project: https://web.archive.org/web/20170702024706/https://perspectives-project.org/
Publishing Keys
^^^^^^^^^^^^^^^
