Consider `html+erb` as a valid language for snippets

To highlight ERB snippets, both `erb` and `html+erb` are used as a
language passed to the code block. However, only the former will be
correctly highlighted.

guides/rails_guides/markdown/renderer.rb

@@ -48,7 +48,7 @@ def brush_for(code_type)
           case code_type
             when 'ruby', 'sql', 'plain'
               code_type
-            when 'erb'
+            when 'erb', 'html+erb'
               'ruby; html-script: true'
             when 'html'
               'xml' # HTML is understood, but there are .xml rules in the CSS