Fix migration class name

Fix migration class name

diff --git a/db/migrate/20190625085735_add_vote_count_index_to_topic_custom_fields.rb b/db/migrate/20190625085735_add_vote_count_index_to_topic_custom_fields.rb
index e968d5a..272906b 100644
--- a/db/migrate/20190625085735_add_vote_count_index_to_topic_custom_fields.rb
+++ b/db/migrate/20190625085735_add_vote_count_index_to_topic_custom_fields.rb
@@ -1,4 +1,4 @@
-class AddVoteCountIndexToPostCustomFields < ActiveRecord::Migration[5.2]
+class AddVoteCountIndexToTopicCustomFields < ActiveRecord::Migration[5.2]
   def change
     execute <<~SQL
       DELETE FROM topic_custom_fields f

GitHub sha: 98790e3c