Suggest chmod 755 instead of 644

Previous suggestion would clear executable bit, meaning directory
couldn't be entered. Fixes #11003.

Test Plan: Read message.

Reviewers: austin, thomie, dfeuer

Reviewed By: thomie, dfeuer

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1350

GHC Trac Issues: #11003