'--user-dir' documentation (correct) (#447)

Summary:
Command line option --user-dir documented in docs/overview.rst
Pull Request resolved: https://github.com/pytorch/fairseq/pull/447

Differential Revision: D13674744

Pulled By: myleott

fbshipit-source-id: 17049ee5c9f692f5298ef9fa7381ee583f269cde