Update css3-box-shadow.html

overflow:hidden，会干掉阴影

Author: Cloudyan

cnDocs/solutions/css3-box-shadow.html

@@ -21,7 +21,6 @@
     position:absolute;
     bottom:-29px;
     left:0;
-    overflow:hidden;
     background:#fff;
     width:100%;
     height:15px;