Docs: remove a duplicated "be" in an accordion demo

Closes gh-2233

Signed-off-by: divdeploy <chenguangxue@outlook.com>

demos/accordion/collapsible.html

@@ -40,7 +40,7 @@ <h3>Section 4</h3>
 </div>
 
 <div class="demo-description">
-<p>By default, accordions always keep one section open. To allow for all sections to be be collapsible, set the <code>collapsible</code> option to true. Click on the currently open section to collapse its content pane.</p>
+<p>By default, accordions always keep one section open. To allow for all sections to be collapsible, set the <code>collapsible</code> option to true. Click on the currently open section to collapse its content pane.</p>
 </div>
 </body>
 </html>