fixes HTML error, p is not allowed as a child of span (#242)

This affected post previews. Changing span for div fixes the issue.

Author: jaimeiniesta

lib/school_house_web/templates/post/_post_preview.html.heex

@@ -9,8 +9,8 @@
     <% end %>
   </div>
   <%= link(to: Routes.post_path(@conn, :show, @post.slug), class: "block mt-4 prose dark:prose-dark") do %>
-    <span class="text-xl font-semibold"><%= raw @post.title %></span>
-    <span class="mt-3 text-base"><%= raw @post.excerpt %></span>
+    <div class="text-xl font-semibold"><%= raw @post.title %></div>
+    <div class="mt-3 text-base"><%= raw @post.excerpt %></div>
   <% end %>
   <div class="flex items-center mt-6">
     <div class="flex-shrink-0">