Merge pull request #32395 from yhirano55/use_oxford_comma_style_in_guide

[ci skip] Use Oxford comma style in guide

guides/source/_welcome.html.erb

@@ -23,6 +23,6 @@ The guides for earlier releases:
 <a href="http://guides.rubyonrails.org/v4.0/">Rails 4.0</a>,
 <a href="http://guides.rubyonrails.org/v3.2/">Rails 3.2</a>,
 <a href="http://guides.rubyonrails.org/v3.1/">Rails 3.1</a>,
-<a href="http://guides.rubyonrails.org/v3.0/">Rails 3.0</a> and
+<a href="http://guides.rubyonrails.org/v3.0/">Rails 3.0</a>, and
 <a href="http://guides.rubyonrails.org/v2.3/">Rails 2.3</a>.
 </p>