fix: missing proxy argument (#5778)

Closes #5761

Author: RobbinBaauw

packages/@vue/cli/bin/vue.js

@@ -62,7 +62,7 @@ program
   .option('-f, --force', 'Overwrite target directory if it exists')
   .option('--merge', 'Merge target directory if it exists')
   .option('-c, --clone', 'Use git clone when fetching remote preset')
-  .option('-x, --proxy', 'Use specified proxy when creating project')
+  .option('-x, --proxy <proxyUrl>', 'Use specified proxy when creating project')
   .option('-b, --bare', 'Scaffold project without beginner instructions')
   .option('--skipGetStarted', 'Skip displaying "Get started" instructions')
   .action((name, cmd) => {