encoding.c: fix rdoc of `__FILE__`

* encoding.c (rb_enc_default_internal): fix rdoc.  `__FILE__` is
  in filesystem encoding but not `default_internal`.
  [ruby-core:61894] [Bug #9713]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45539 b2dd03c8-39d4-4d8f-98ff-823fe69b080e