Add missing column on "Changing Column Types"

What does this MR do?

Step 2 of "Changing column types" is missing the column on the example. So this MR adds it.

As a reference, cleanup_concurrent_column_type_change on migration_helpers.rb uses two arguments: table and column

def cleanup_concurrent_column_type_change(table, column)
  temp_column = "#{column}_for_type_change"

   transaction do
    # ...
   end
end

Related issues

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