Improve error message from ArcanistXHPASTLinter::LINT_FORMATTED_STRING

Summary: Currently the error message from ArcanistXHPASTLinter::LINT_FORMATTED_STRING is slightly wrong, mentioning xsprintf instead of the actual sprintf-style function which was used. This diff changes the error message to be more correct.

Test Plan: Linted a file containing sprintf('%s') and saw a reasonable lint message.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12871