Improved example setting for differential.generated-paths

Summary: This is to make it more obvious how to ignore a folder that may be at the root, and it resolves https://secure.phabricator.com/T8894

Test Plan: Just a documentation change

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, nornagon

Differential Revision: https://secure.phabricator.com/D14193