Update the text-limit single migration example

What does this MR do and why?

The example migration for adding a text column removes the text-limit constraint in the down method:

def down
    remove_text_limit :sprints, :extended_title
    with_lock_retries do
      remove_column :sprints, :extended_title, if_exists: true
    end
end

Removing the text limit separately does not seem to be necessary.