Use proper path separators for Display file

This change fixes an issue with the compilation server on OSX where completion wasn't working properly. 

An alternative and slightly simpler fix would be to use Filename.dir_sep, but that relies on ocaml version 3.11.2. On Windows the win-msvc binary version is at 3.11.0 and I'm not sure if people are using a newer version of ocaml on Windows.