fix: specify wss should be from a node provider (#1034)

Author: wlawt

docs/base-chain/flashblocks/apps.mdx

@@ -214,7 +214,8 @@ Subscribe to real-time streams of Flashblocks data via WebSocket. Three subscrip
 import WebSocket from 'ws';
 import { createHash } from 'crypto';
 
-const ws = new WebSocket('wss://sepolia.flashblocks.base.org/ws');
+// `ws` should be a WebSocket RPC URL, not from the Flashblock WebSocket Stream
+const ws = new WebSocket('wss://my-node.node-provider.com/ws');
 
 ws.on('open', () => {
   console.log('Connected to Flashblocks WebSocket');