Change PHP keywords to comply with PSR2

bgmrequested to merge
github/fork/CodeLingoBot/rewrite-civicrm-org-civicrm-module-cividiscount-psr2-lower-case-constant-values into master

Created by: mullikine

Hi, I've changed a PHP keyword to comply with this standard in PSR2. It's admittedly a small fix but I hope it helps!

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