docs: fix mismatched variable names

resolve #1802

vitejs/vite@76082e3 の反映です。

Author: shuuji3

guide/api-environment-runtimes.md

@@ -304,28 +304,28 @@ function createWorkerEnvironment(name, config, context) {
   const handlerToWorkerListener = new WeakMap()
 
   const workerHotChannel = {
-    send: (data) => w.postMessage(data),
+    send: (data) => worker.postMessage(data),
     on: (event, handler) => {
       if (event === 'connection') return
 
       const listener = (value) => {
         if (value.type === 'custom' && value.event === event) {
           const client = {
             send(payload) {
-              w.postMessage(payload)
+              worker.postMessage(payload)
             },
           }
           handler(value.data, client)
         }
       }
       handlerToWorkerListener.set(handler, listener)
-      w.on('message', listener)
+      worker.on('message', listener)
     },
     off: (event, handler) => {
       if (event === 'connection') return
       const listener = handlerToWorkerListener.get(handler)
       if (listener) {
-        w.off('message', listener)
+        worker.off('message', listener)
         handlerToWorkerListener.delete(handler)
       }
     },