Fix comment in field specification

Test Plan: Logged type of 'arc:unit'.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3565

src/applications/differential/field/specification/DifferentialFieldSpecification.php

@@ -908,7 +908,7 @@ abstract class DifferentialFieldSpecification {
    * the key from @{method:getRequiredDiffProperties}.
    *
    * @param  string      Diff property key.
-   * @return string|null Diff property, or null if the property does not have
+   * @return mixed|null  Diff property, or null if the property does not have
    *                     a value.
    * @task context
    */