fix checkbox rendering in markdown

Author: wxiaoguang

web_src/less/markup/content.less

@@ -158,9 +158,12 @@
 
   .task-list-item {
     list-style-type: none;
+    position: relative;
 
     input[type="checkbox"] {
-      margin: 0 6px .25em -1.6em;
+      position: absolute;
+      top: .25em;
+      left: -1.6em;
     }
   }