doc(ios): update swift-tools-version to 5.9 in iOS Plugin Development Guide

[GitToTheHub]

• Was changed in chore: set swift-tools-version to 5.9 cordova-ios#1611: Since we require Xcode 15.0 or newer, we can safely set the swift-tools-version to 5.9.

Platforms affected

Motivation and Context

Description

Testing

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary