wiki update: iOS Screen Sharing instruction missing info.plist update

[mokuzuu]

Describe the bug
In order to make screen sharing work on example app, I needed to add RTCAppGroupIdentifier to iOS/BroadcastExtension/info.plist and iOS/info.plist

SDK Version
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iOS/macOS Version
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Xcode Version
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(Please provide the Swift version if you know it)

Steps to Reproduce
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Expected behavior
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