Fix another "error_explanation" css class in guides [ci skip]

guides/code/getting_started/app/views/posts/_form.html.erb

 <%= form_for @post do |f| %>
   <% if @post.errors.any? %>
-  <div id="errorExplanation">
+  <div id="error_explanation">
     <h2><%= pluralize(@post.errors.count, "error") %> prohibited
       this post from being saved:</h2>
     <ul>
@@ -14,12 +14,12 @@
     <%= f.label :title %><br>
     <%= f.text_field :title %>
   </p>
- 
+
   <p>
     <%= f.label :text %><br>
     <%= f.text_area :text %>
   </p>
- 
+
   <p>
     <%= f.submit %>
   </p>