Fix modal close button

lms/static/sass/shared/_modal.scss

@@ -282,7 +282,7 @@
       position: absolute;
       right: 2px;
       top: 0px;
-      z-index: 3;
+      z-index: 100;
       color: $lighter-base-font-color;
       font: normal 1.2rem/1.2rem $sans-serif;
       text-align: center;