Fix usage docs around modals

Author: jcoyne

app/javascript/blacklight/modal.js

@@ -44,7 +44,7 @@
 
       <div class="modal-body">
         <p>Some message</p>
-        <%= link_to "This result will still be within modal", some_link, data: { blacklight: "preserve" } %>
+        <%= link_to "This result will still be within modal", some_link, data: { blacklight_modal: "preserve" } %>
       </div>