Fix incorrect linter code being raised

Summary: `ArcanistXHPASTLinter::lintSpaceAroundConcatenationOperators` should raise `LINT_CONCATENATION_OPERATOR`, not `LINT_BINARY_EXPRESSION_SPACING`.

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10553