pp: fix the width and indentation of second and later line contents

[makenowjust]

For example,

class X
  def pretty_print(pp : PrettyPrint)
    100.times do
      pp.group do
        pp.text "x"
        pp.breakable
      end
    end
  end
end

pp X.new

Above code prints this:

X.new # => x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
           x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
           x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
           x x x x x x x

This second and third lines are shorter than first line, it seems odd.

Expected result:

X.new # => x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
           x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
           x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x

This PR fixes pp and PrettyPrint to output correctly.