Fix indentation in migration template file

This PR fixes indentation of migration files that would be generated with `rails generate groonga:migration`.

When name of migration file starts with 'add' or 'remove', migration outputs "change_table" statement. Indentation of the statement isn't indented property (see below).

```ruby
class AddSomeToFoo < ActiveGroonga::Migration
  def up
  change_table(:foo) do |table|
  end
  end
  # snip
end