refactor: add CIcon example in _nav.js

Author: woothu

src/containers/_nav.js

@@ -1,9 +1,12 @@
+import React from 'react'
+import CIcon from '@coreui/icons-react'
+
 export default [
   {
     _tag: 'CSidebarNavItem',
     name: 'Dashboard',
     to: '/dashboard',
-    icon: 'cil-speedometer',
+    icon: <CIcon name="cil-speedometer" customClasses="c-sidebar-nav-icon"/>,
     badge: {
       color: 'info',
       text: 'NEW',