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