Fix typo in php.md

[thelinuxfoundation]

Thank you for your pull request. Before we can look at your contribution, we need to ensure all contributors are covered by a Contributor License Agreement.

After the following items are addressed, please respond with a new comment here, and the automated system will re-verify.

• User @tomascassidy isn't covered by a CLA. They will need to complete the form at https://identity.linuxfoundation.org/projects/cncf

Regards, CLA GitHub bot

[wwuck]

CLA

[carl-mastrangelo]

@tomascassidy merged, thanks!