docs: update site title from 'VuePress 1.x' to 'VuePress'

See: #1737

Author: ulivz

packages/docs/docs/.vuepress/config.js

@@ -5,12 +5,12 @@ module.exports = ctx => ({
   locales: {
     '/': {
       lang: 'en-US',
-      title: 'VuePress 1.x',
+      title: 'VuePress',
       description: 'Vue-powered Static Site Generator'
     },
     '/zh/': {
       lang: 'zh-CN',
-      title: 'VuePress 1.x',
+      title: 'VuePress',
       description: 'Vue 驱动的静态网站生成器'
     }
   },