Fix value of CSS background-color property in Rails guide

Change background-color value to transparent, which is the default value of background-color.

[ci skip]

guides/assets/stylesheets/syntaxhighlighter/shThemeRailsGuides.css

@@ -90,7 +90,7 @@
 }
 .syntaxhighlighter .script {
   color: #222 !important;
-  background-color: none !important;
+  background-color: transparent !important;
 }
 .syntaxhighlighter .color1, .syntaxhighlighter .color1 a {
   color: gray !important;