dbm.c: fix doc

* ext/dbm/dbm.c (Init_dbm): [DOC] as UNIX permissions are octal
  numbers, needs to be prefixed by 0.  [Fix GH-800]
[ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49065 b2dd03c8-39d4-4d8f-98ff-823fe69b080e