Update testing documentation

Current versions of Thinking Sphinx use `indices_location`, so change the example code to use that instead of `searchd_file_path`.

Also, change the example to use the `index` method defined in `SphinxHelpers`, and use `type: :feature` instead of `type: :request` to reflect current RSpec conventions.

Note: I attempted to make this change through Gyoza's edit function, but it looks like the pull request never actually got created (I waited >10 minutes). I'm recreating it manually here; apologies if it ends up being a duplicate.