update engines guide to explicitly reference relative-to-engine-root-path for db/seeds.rb

railties/lib/rails/engine.rb

@@ -326,7 +326,7 @@ module Rails
   # migration in the application and rerun copying migrations.
   #
   # If your engine has migrations, you may also want to prepare data for the database in
-  # the <tt>seeds.rb</tt> file. You can load that data using the <tt>load_seed</tt> method, e.g.
+  # the <tt>db/seeds.rb</tt> file. You can load that data using the <tt>load_seed</tt> method, e.g.
   #
   #   MyEngine::Engine.load_seed
   #