Update shard index (added shard 21)

shard_index.json

@@ -17,6 +17,12 @@
       "total_shards": 22,
       "num_sequences": 1015826,
       "block_size": 1024
+    },
+    "21": {
+      "shard_id": 21,
+      "total_shards": 22,
+      "num_sequences": 1015846,
+      "block_size": 1024
     }
   },
   "total_shards": 22