Use symbol form of index name

All the other indexes that use an explicit (non-generated) name use
symbols, except this one. This is for consistency. I've tested
that a version of this gem that includes this change, generates the Good
Jobs migration, and that the migration applies as expected.

lib/generators/good_job/templates/install/migrations/create_good_jobs.rb.erb

@@ -70,7 +70,7 @@ class CreateGoodJobs < ActiveRecord::Migration<%= migration_version %>
       t.index :key, unique: true
     end
 
-    add_index :good_jobs, :scheduled_at, where: "(finished_at IS NULL)", name: "index_good_jobs_on_scheduled_at"
+    add_index :good_jobs, :scheduled_at, where: "(finished_at IS NULL)", name: :index_good_jobs_on_scheduled_at
     add_index :good_jobs, [:queue_name, :scheduled_at], where: "(finished_at IS NULL)", name: :index_good_jobs_on_queue_name_and_scheduled_at
     add_index :good_jobs, [:active_job_id, :created_at], name: :index_good_jobs_on_active_job_id_and_created_at
     add_index :good_jobs, :concurrency_key, where: "(finished_at IS NULL)", name: :index_good_jobs_on_concurrency_key_when_unfinished