Update SwiftPackageManager.md (#7273)
Update SwiftPackageManager.md (#7273)

Add a missing '/' into the run srcipt for Swift Package Manager installation. I needed it for my build.
firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk