[Doc] Recover example about Queue

`trunk@42862` dropped example's last line.

https://github.com/ruby/ruby/commit/e334bb2ce5d8876b020ab681f21595e2e1c9d601#diff-8783a9b452e430bcf0d7b0c6e34f1db0L144
https://github.com/ruby/ruby/commit/e334bb2ce5d8876b020ab681f21595e2e1c9d601#diff-38e7b9d781319cfbc49445f8f6625b8aR195

This brings no output.

```queue_example1.rb
queue = Queue.new

producer = Thread.new do
  5.times do |i|
    sleep rand(i) # simulate expense
    queue << i
    puts "#{i} produced"
  end
end

consumer = Thread.new do
  5.times do |i|
    value = queue.pop
    sleep rand(i/2) # simulate expense
    puts "consumed #{value}"
  end
end
```

```queue_example2.rb
queue = Queue.new

producer = Thread.new do
  5.times do |i|
    sleep rand(i) # simulate expense
    queue << i
    puts "#{i} produced"
  end
end

consumer = Thread.new do
  5.times do |i|
    value = queue.pop
    sleep rand(i/2) # simulate expense
    puts "consumed #{value}"
  end
end

consumer.join
```

$ ruby queue_example1.rb
$

$ ruby queue_example2.rb
0 produced
1 produced
consumed 0
consumed 1
2 produced
consumed 2
3 produced
consumed 3
4 produced
consumed 4
$

Co-Authored-By: Sanemat <o.gata.ken@gmail.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64058 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

1 files changed, 2 insertions, 0 deletions

diff --git a/thread_sync.c b/thread_sync.c
index 1624c470a5..279c644093 100644
--- a/thread_sync.c
+++ b/thread_sync.c
@@ -737,6 +737,8 @@ queue_closed_result(VALUE self, struct rb_queue *q)
  *	  end
  *	end
  *
+ *	consumer.join
+ *
  */
 
 /*