minor #6645 Fix bootstrap class name help-block (foaly-nr1)

This PR was merged into the 2.7 branch.

Discussion
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Fix bootstrap class name help-block

I checked the [bootstrap documentation](http://getbootstrap.com/css/#forms-example) as well as the bootstrap source code, there is no such class as `help`.

Commits
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af5a5be Fix bootstrap class name help-block

Author: xabbuh

cookbook/form/form_customization.rst

@@ -1055,7 +1055,7 @@ form, modify the ``use`` tag and add the following:
         {{ block('base_form_widget_simple') }}
 
         {% if help is defined %}
-            <span class="help">{{ help }}</span>
+            <span class="help-block">{{ help }}</span>
         {% endif %}
     {% endblock %}
 
@@ -1070,7 +1070,7 @@ the following:
         {{ parent() }}
 
         {% if help is defined %}
-            <span class="help">{{ help }}</span>
+            <span class="help-block">{{ help }}</span>
         {% endif %}
     {% endblock %}