Fix best_beacon_node property on MultiBeaconNode - fix ordering (#8)

src/providers/multi_beacon_node.py

@@ -138,7 +138,7 @@ async def __aexit__(self, exc_type, exc_val, exc_tb):
     def best_beacon_node(self):
         return next(
             bn
-            for bn in sorted(self.beacon_nodes, key=lambda bn: bn.score)
+            for bn in sorted(self.beacon_nodes, key=lambda bn: bn.score, reverse=True)
             if bn.initialized
         )