[FIX] website_slides: add gray background to lesson not fullscreen display

Simple fix to already have the standard gray background. Future commits will
improve display of not fullscreen lessons but this fix was necessary for
incoming work.

Commit linked to task ID 1941250 and PR #31512.

addons/website_slides/views/website_slides_templates_lesson.xml

@@ -8,6 +8,7 @@
             <t t-call="website_slides.course_nav">
                 <t t-set="channel" t-value="slide.channel_id"/>
             </t>
+        <div class="wrap o_wslides_wrap">
             <div class="o_wslides_slide_header">
                 <div class="o_wslides_slide_header_container">
                     <div class="o_wslides_slide_header_box">
@@ -416,6 +417,7 @@
                 </div>
             </div>
         </div>
+        </div>
     </t>
 </template>