Studio: revises subtitle of Textbooks view to be consistent with others

cms/templates/textbooks.html

@@ -49,7 +49,7 @@ $(function() {
   <div class="wrapper-mast wrapper">
     <header class="mast has-actions has-subtitle">
       <h1 class="page-header">
-        <small class="subtitle">${_("Course Content")}</small>
+        <small class="subtitle">${_("Content")}</small>
         <span class="sr">&gt; </span>${_("Textbooks")}
       </h1>