help avoid misuse of Carousel's .active when copy-pasting from examples; fix #13252

docs/_includes/js/carousel.html

@@ -46,6 +46,12 @@
         ...
       </div>
     </div>
+    <div class="item">
+      <img src="..." alt="...">
+      <div class="carousel-caption">
+        ...
+      </div>
+    </div>
     ...
   </div>
 
@@ -105,7 +111,7 @@
     </div>
   </div><!-- /example -->
 {% highlight html %}
-<div class="item active">
+<div class="item">
   <img src="..." alt="...">
   <div class="carousel-caption">
     <h3>...</h3>