fix pluralization in doc [ci skip]

guides/source/active_record_migrations.md

@@ -477,7 +477,7 @@ column names can not be derived from the table names, you can use the
 `:column` and `:primary_key` options.
 
 Rails will generate a name for every foreign key starting with
-`fk_rails_` followed by 10 character which is deterministically
+`fk_rails_` followed by 10 characters which are deterministically
 generated from the `from_table` and `column`.
 There is a `:name` option to specify a different name if needed.