Fix type spec for 'coverage'

Summary: Ref T8670. The spec for "coverage" is incorrect, it's a map of paths to coverage information.

Test Plan: See next diff.

Reviewers: joshuaspence, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8670

Differential Revision: https://secure.phabricator.com/D13435