Fix typo in guide: _articles should be _article

The guide contains a typo in the "local_assigns" section, where
rendering a model named 'Article' via `render @articles` is shown to
render a partial called `_articles.html.erb`, when in fact the necessary
partial name is `_article.html.erb`

guides/source/layouts_and_rendering.md

@@ -1155,7 +1155,7 @@ To pass a local variable to a partial in only specific cases use the `local_assi
   <%= render article, full: true %>
   ```
 
-* `_articles.html.erb`
+* `_article.html.erb`
 
   ```erb
   <h2><%= article.title %></h2>