Merge pull request #1808 from yasslab/apply_syntax_highlight_to_shell

shell のコード部分にシンタックスハイライトを適用

Author: rakuda-san-desu

guides/rails_guides/markdown/renderer.rb

@@ -69,7 +69,7 @@ def brush_for(code_type)
             "ERB"
           when "html"
             "XML" # HTML is understood, but there are .xml rules in the CSS
-          when "bash"
+          when "bash", "shell"
             "ConsoleLexer"
           else
             "PlainText"