fix($theme-default): regression of custom container default title (#1875)

Author: meteorlxy

packages/@vuepress/theme-default/index.js

@@ -24,18 +24,21 @@ module.exports = (options, ctx) => ({
     ['container', {
       type: 'tip',
       defaultTitle: {
+        '/': 'TIP',
         '/zh/': '提示'
       }
     }],
     ['container', {
       type: 'warning',
       defaultTitle: {
+        '/': 'WARNING',
         '/zh/': '注意'
       }
     }],
     ['container', {
       type: 'danger',
       defaultTitle: {
+        '/': 'WARNING',
         '/zh/': '警告'
       }
     }]