chore: fix serve command name

Author: yyx990803

docs/cli-service.md

@@ -28,7 +28,7 @@ This is what you will see in the `package.json` of a project using the default p
 ### serve
 
 ```
-Usage: vue-cli-service dev [options]
+Usage: vue-cli-service serve [options]
 
 Options:
 

packages/@vue/cli-service/lib/commands/serve.js

@@ -15,7 +15,7 @@ const defaults = {
 module.exports = (api, options) => {
   api.registerCommand('serve', {
     description: 'start development server',
-    usage: 'vue-cli-service dev [options]',
+    usage: 'vue-cli-service serve [options]',
     options: {
       '--open': `open browser on server start`,
       '--mode': `specify env mode (default: ${defaults.mode})`,