[DOC] Use File.open with block instead of File.new without close [ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61433 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

io.c

@@ -5175,8 +5175,9 @@ internal_pwrite_func(void *ptr)
  *  Raises <code>SystemCallError</code> on error and <code>NotImplementedError</code>
  *  if platform does not implement the system call.
  *
- *     f = File.new("out", "w")
+ *     File.open("out", "w") do |f|
  *       f.pwrite("ABCDEF", 3)   #=> 6
+ *     end
  *
  *     File.read("out")          #=> "\u0000\u0000\u0000ABCDEF"
  */