[DOC] fixed pid shown in the doc comment on fork

Merged: https://github.com/ruby/ruby/pull/15988

Merged-By: nobu <nobu@ruby-lang.org>

process.c

@@ -4233,7 +4233,7 @@ rb_proc__fork(VALUE _obj)
  *    puts "Before the fork: #{Process.pid}"
  *    fork do
  *      puts "In the child process: #{Process.pid}"
- *    end                   # => 382141
+ *    end                   # => 420520
  *    puts "After the fork: #{Process.pid}"
  *
  *  Output: