Fix generating incorrect accessibility by prototype rb

[pocke]

Problem

I implemented generating accessibility methods in #481 , but it has a bug 🙇‍♂️
It generates incorrect accessibility when methods are continually defined with multiple private def foo() end syntax.

For example:

class C
  private def prv1() end
  private def prv2() end
  def pub() end
end

$ exe/rbs prototype rb test.rb
class C
  private

  def prv1: () -> nil

  def prv2: () -> nil

  def pub: () -> nil
end

it needs public before def pub: () -> nil, but there isn't.

Cause

I misunderstood the usage of Array#slice. It needs to receive the size of the array if I need the entire of the array, but I passed the size - 1.

So this pull request replaces size - 1 with just Array#size

[soutaro]

👍