Add missing type="button" attrs to <button>s in list-group docs

Fixes Bootlint warnings:
	W007 Found one or more `<button>`s missing a `type` attribute.

[skip sauce]

docs/_includes/components/list-group.html

@@ -80,20 +80,20 @@
   <p>List group items may be buttons instead of list items (that also means a parent <code>&lt;div&gt;</code> instead of an <code>&lt;ul&gt;</code>). No need for individual parents around each element. <strong class="text-danger">Don't use the standard <code>.btn</code> classes here.</strong></p>
   <div class="bs-example" data-example-id="list-group-btns">
     <div class="list-group">
-      <button class="list-group-item">Cras justo odio</button>
-      <button class="list-group-item">Dapibus ac facilisis in</button>
-      <button class="list-group-item">Morbi leo risus</button>
-      <button class="list-group-item">Porta ac consectetur ac</button>
-      <button class="list-group-item">Vestibulum at eros</button>
+      <button type="button" class="list-group-item">Cras justo odio</button>
+      <button type="button" class="list-group-item">Dapibus ac facilisis in</button>
+      <button type="button" class="list-group-item">Morbi leo risus</button>
+      <button type="button" class="list-group-item">Porta ac consectetur ac</button>
+      <button type="button" class="list-group-item">Vestibulum at eros</button>
     </div>
   </div>
 {% highlight html %}
 <div class="list-group">
-  <button class="list-group-item">Cras justo odio</button>
-  <button class="list-group-item">Dapibus ac facilisis in</button>
-  <button class="list-group-item">Morbi leo risus</button>
-  <button class="list-group-item">Porta ac consectetur ac</button>
-  <button class="list-group-item">Vestibulum at eros</button>
+  <button type="button" class="list-group-item">Cras justo odio</button>
+  <button type="button" class="list-group-item">Dapibus ac facilisis in</button>
+  <button type="button" class="list-group-item">Morbi leo risus</button>
+  <button type="button" class="list-group-item">Porta ac consectetur ac</button>
+  <button type="button" class="list-group-item">Vestibulum at eros</button>
 </div>
 {% endhighlight %}