Fixing examples to use Sha512 with Ristretto instead of Sha256 (#122)

examples/simple_login.rs

@@ -41,7 +41,7 @@ struct Default;
 impl CipherSuite for Default {
     type Group = curve25519_dalek::ristretto::RistrettoPoint;
     type KeyExchange = opaque_ke::key_exchange::tripledh::TripleDH;
-    type Hash = sha2::Sha256;
+    type Hash = sha2::Sha512;
     type SlowHash = opaque_ke::slow_hash::NoOpHash;
 }