Fixed plus sign icon for overall grade range add button.

TNL-890

cms/templates/settings_graders.html

@@ -49,7 +49,7 @@
           <ol class="list-input">
             <li class="field" id="field-course-grading-range">
               <div class="grade-controls course-grading-range well">
-                <a href="#" class="new-grade-button"><span class="plus-icon"></span></a>
+                <a href="#" class="new-grade-button"><span class="plus-icon icon-plus"></span></a>
                 <div class="grade-slider" dir="ltr">
                   <div class="grade-bar">
                     <ol class="increments">