Update sytle for code (#81)

https://github.com/rails/rails/blob/c453a5fa88e89bc0bc8efde3b397f2464712bb49/guides/assets/stylesheets/main.css#L37-L41

Author: toshimaru

src/_sass/railsdoc.scss

@@ -15,7 +15,10 @@ a {
 }
 
 code {
-  color: #333;
+  color: #222;
+  background: #eee;
+  border-radius: 2px;
+  padding: 1px 3px;
 }
 
 // Header