plugin.md: Exclude CocoaPod when using Swift Package Manager

[GitToTheHub]

• Document the use of nospm="true" to exclude Pods in cordova-ios 8 and newer when Swift Package Manager is used
• Write Cordova-iOS as cordova-ios since this is the package name

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