DOCSP-44610: fix php bson link

[rustagir]

Pull Request Info

PR Reviewing Guidelines

JIRA - https://jira.mongodb.org/browse/DOCSP-44610
Staging - https://deploy-preview-175--docs-php-library.netlify.app/reference/method/MongoDBCollection-count/#behavior

Self-Review Checklist

• Is this free of any warnings or errors in the RST?
• Did you run a spell-check?
• Did you run a grammar-check?
• Are all the links working?
• Are the facets and meta keywords accurate?

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