Fix index example in Using Sprockets doc
Fix index example in Using Sprockets doc

opal/opal,opal/opal,iliabylich/opal,iliabylich/opal,Mogztter/opal,iliabylich/opal,Mogztter/opal,Mogztter/opal,kachick/opal,kachick/opal,opal/opal,Mogztter/opal,opal/opal,kachick/opal,kachick/opal