fix exist typo

app/models/user_detail.rb

@@ -19,7 +19,7 @@ class UserDetail < ApplicationRecord
 
   # For backward compatibility.
   # Older migrations (and their tests) reference the `User.migration_bot` where the `bio` attribute is set.
-  # Here we disable writing the markdown cache when the `bio_html` column does not exists.
+  # Here we disable writing the markdown cache when the `bio_html` column does not exist.
   override :invalidated_markdown_cache?
   def invalidated_markdown_cache?
     self.class.column_names.include?('bio_html') && super