Reword alert and list group text to avoid "a/an" problems

Closes https://github.com/twbs/bootstrap/issues/25611

docs/4.0/components/alerts.md

@@ -13,7 +13,7 @@ Alerts are available for any length of text, as well as an optional dismiss butt
 {% example html %}
 {% for color in site.data.theme-colors %}
 <div class="alert alert-{{ color.name }}" role="alert">
-  This is a {{ color.name }} alert—check it out!
+  A simple {{ color.name }} alert—check it out!
 </div>{% endfor %}
 {% endexample %}
 
@@ -27,7 +27,7 @@ Use the `.alert-link` utility class to quickly provide matching colored links wi
 {% example html %}
 {% for color in site.data.theme-colors %}
 <div class="alert alert-{{ color.name }}" role="alert">
-  This is a {{ color.name }} alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
+  A simple {{ color.name }} alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
 </div>{% endfor %}
 {% endexample %}
 

docs/4.0/components/list-group.md

@@ -103,7 +103,7 @@ Use contextual classes to style list items with a stateful background and color.
   <li class="list-group-item">Dapibus ac facilisis in</li>
 
   {% for color in site.data.theme-colors %}
-  <li class="list-group-item list-group-item-{{ color.name }}">This is a {{ color.name }} list group item</li>{% endfor %}
+  <li class="list-group-item list-group-item-{{ color.name }}">A simple {{ color.name }} list group item</li>{% endfor %}
 </ul>
 {% endexample %}
 
@@ -114,7 +114,7 @@ Contextual classes also work with `.list-group-item-action`. Note the addition o
   <a href="#" class="list-group-item list-group-item-action">Dapibus ac facilisis in</a>
 
   {% for color in site.data.theme-colors %}
-  <a href="#" class="list-group-item list-group-item-action list-group-item-{{ color.name }}">This is a {{ color.name }} list group item</a>{% endfor %}
+  <a href="#" class="list-group-item list-group-item-action list-group-item-{{ color.name }}">A simple {{ color.name }} list group item</a>{% endfor %}
 </div>
 {% endexample %}