Asset Pipeline: add 'the'

Some guides work without 'the'. For instance, 'Migrations' or
'Form Helpers.' But when we talk about the asset pipeline, we...
always say 'the asset pipeline.' The guide title should reflect this.

guides/source/asset_pipeline.md

-Asset Pipeline
-==============
+The Asset Pipeline
+==================
 
 This guide covers the asset pipeline introduced in Rails 3.1.
 By referring to this guide you will be able to: