Fix a duplicate case statement

Summary: This appears to be a typo, identified by ArcanistXHPASTLinter::LINT_DUPLICATE_SWITCH_CASE (see D11171).

Test Plan: arc lint

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11194