fixed another typo in edge guides

guides/source/getting_started.textile

@@ -1191,7 +1191,7 @@ delete "posts/:id" => "posts#destroy"
 
 That's a lot to type for covering a single *resource*. Fortunately,
 Rails provides a +resources+ method which can be used to declare a
-standard REST resource. Here's how +config/routes/rb+ looks after the
+standard REST resource. Here's how +config/routes.rb+ looks after the
 cleanup:
 
 <ruby>