Change deprecation warning for export_file
Change deprecation warning for export_file

Summary: Filegroup was never really fully implemented usefully for individual files. Consider export_file to be okay for now for single files.

Reviewed By: mhorowitz, cjhopman

shipit-source-id: 61328b11ac

JoelMarcey/buck,zpao/buck,nguyentruongtho/buck,nguyentruongtho/buck,JoelMarcey/buck,nguyentruongtho/buck,kageiit/buck,zpao/buck,kageiit/buck,Addepar/buck,facebook/buck,facebook/buck,Addepar/buck,Addepar/buck,facebook/buck,JoelMarcey/buck,zpao/buck,nguyentruongtho/buck,Addepar/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,facebook/buck,kageiit/buck,Addepar/buck,zpao/buck,kageiit/buck,facebook/buck,nguyentruongtho/buck,Addepar/buck,zpao/buck,nguyentruongtho/buck,Addepar/buck,kageiit/buck,JoelMarcey/buck,JoelMarcey/buck,nguyentruongtho/buck,JoelMarcey/buck,zpao/buck,facebook/buck,Addepar/buck,Addepar/buck,Addepar/buck,JoelMarcey/buck,facebook/buck,JoelMarcey/buck,JoelMarcey/buck,kageiit/buck,Addepar/buck,zpao/buck,kageiit/buck,Addepar/buck,Addepar/buck