io.c: fix rdoc of append mode

* io.c (rb_io_initialize): [DOC] fix rdoc of append mode.  it does
  not move the pointer at open.  [ruby-core:63747] [Bug #10039]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46876 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-07-20

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46877 b2dd03c8-39d4-4d8f-98ff-823fe69b080e