fix($theme-default): improve last-updated text color contrast (#2282)

Author: cyrilf

packages/@vuepress/theme-default/components/PageEdit.vue

@@ -129,7 +129,7 @@ export default {
       color lighten($textColor, 25%)
     .time
       font-weight 400
-      color #aaa
+      color #767676
 
 @media (max-width: $MQMobile)
   .page-edit