Use tire.index.name instead of index_name, as user could be using a proc (tire already handles that)

I was trying to implement dynamic index names with Searchkick today (for a multi-tenanted app) and ran into this slight bug. Tire already supports having the index_name as a Proc, but there was this part of the code that used index_name over tire.index.name. Doing so will crash if the index_name is indeed a proc. Thanks!