Update offline page

assets/javascripts/app/config.coffee.erb

@@ -14,4 +14,4 @@ app.config =
   release: <%= Time.now.utc.httpdate.to_json %>
   mathml_stylesheet: '<%= App.cdn_origin %>/mathml.css'
   service_worker_path: '/service-worker.js'
-  service_worker_enabled: <%= App.environment == 'production' || !ENV["ENABLE_SERVICE_WORKER"].nil? %>
+  service_worker_enabled: <%= App.environment == 'production' || ENV["ENABLE_SERVICE_WORKER"] == 'true' %>

assets/javascripts/templates/pages/offline_tmpl.coffee

@@ -47,7 +47,7 @@ canICloseTheTab = ->
     reason = "aren't available in your browser (or are disabled)"
 
     if app.config.env != 'production'
-      reason = "are disabled in your development instance of DevDocs (enable them by setting the ENABLE_SERVICE_WORKERS environment variable)"
+      reason = "are disabled in your development instance of DevDocs (enable them by setting the <code>ENABLE_SERVICE_WORKERS</code> environment variable to <code>true</code>)"
 
     """ No. Service Workers #{reason}, so loading <a href="//devdocs.io">devdocs.io</a> offline won't work.<br>
         The current tab will continue to function even when you go offline (provided you installed all the documentations beforehand). """