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

src/applications/harbormaster/storage/build/HarbormasterBuildUnitMessage.php

@@ -32,7 +32,7 @@ final class HarbormasterBuildUnitMessage
       'result' => 'string',
       'duration' => 'optional float',
       'path' => 'optional string',
-      'coverage' => 'optional string',
+      'coverage' => 'optional map<string, wild>',
     );
 
     // We're just going to ignore extra keys for now, to make it easier to