Fix minor issues in the Turbo Modules iOS doc

[sepeth]

Hi there,

Main issue was that instead of open TurboModuleExample.xcworkspace, open SampleApp.xcworkspace was used.

NS_ASSUME_NONNULL_END should already be existing in the file, but I made the mistake of clicking the copy button at the top when viewing the docs, and pasting directly into the file. Given that it is just one more line, it wouldn't hurt to have it in the diff.

And, I have a question, I see some copies of this in the website/versioned_docs/**/* as well, should I update those as well, or is there a script/automation that does this?

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