Fix typo in doc for updateSpecificCoupons

[sjb9774]

Description

Just a tiny typo correction in the ResourceModel\Coupon.php:updateSpecificCoupons doc, as "its" is the possessive form of "it" and "it's" is a contraction of "it" and "is".

Manual testing scenarios

N/A

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

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[magento-engcom-team]

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[magento-engcom-team]

Hi @sjb9774. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.2.5 release.