Use h1 from header for guides title tag [ci-skip]

As the main heading is now a h1, the h1 should be used for the title.

Author: p8

guides/rails_guides/markdown.rb

@@ -150,7 +150,7 @@ def generate_index
       end
 
       def generate_title
-        if heading = Nokogiri::HTML.fragment(@header).at(:h2)
+        if heading = Nokogiri::HTML.fragment(@header).at(:h1)
           @title = "#{heading.text} — Ruby on Rails Guides"
         else
           @title = "Ruby on Rails Guides"