update welcome.md

resource/locales/en-US/welcome.md

@@ -3,7 +3,7 @@
 [![GitHub Releases](https://img.shields.io/github/release/weseek/growi.svg)](https://github.com/weseek/growi/releases/latest)
 [![MIT License](https://img.shields.io/badge/license-MIT-blue.svg?style=flat)](LICENSE)
 
-<div class="panel panel-default">
+<div class="panel panel-primary">
   <div class="panel-heading">Tips</div>
   <div class="panel-body"><ul>
     <li>Ctrl(⌘)-/ to show quick help</li>

resource/locales/ja/welcome.md

@@ -3,7 +3,7 @@
 [![GitHub Releases](https://img.shields.io/github/release/weseek/growi.svg)](https://github.com/weseek/growi/releases/latest)
 [![MIT License](https://img.shields.io/badge/license-MIT-blue.svg?style=flat)](LICENSE)
 
-<div class="panel panel-default">
+<div class="panel panel-primary">
   <div class="panel-heading">Tips</div>
   <div class="panel-body"><ul>
     <li>Ctrl(⌘)-/ でショートカットヘルプを表示します</li>