Fix SipHash primitive classification in cryptography registry (#883)

Changes the SipHash registry entry to classify it as `mac` instead of
`hash`, and updates the reference URL to an archival source.


Fixes #882

Author: stevespringett

schema/cryptography-defs.json

@@ -1548,13 +1548,13 @@
       "standard": [
         {
           "name": "SipHash Spec",
-          "url": "https://131002.net/siphash/"
+          "url": "https://doi.org/10.1007/978-3-642-34931-7_28"
         }
       ],
       "variant": [
         {
           "pattern": "SipHash[-{compressionRounds}-{finalizationRounds}]",
-          "primitive": "hash"
+          "primitive": "mac"
         }
       ]
     },