fix minor issues in sample code in README

[bcherry]

I'm running through setup on the various SDKs and noticed that the sample UIKit code here had a couple minor issues that caused errors when pasted as-is

• await in synchronous context (viewDidLoad). I replaced this with a plain Task { ... } which should be fine for sample code
• faulty catch syntax with unnecessary trailing in - I removed the whole let bit because even fixing the syntax still leaves a compile warning about the unused variable error.
• unnecessary ? on non-optional variables - removed
• implicitly captured self within a closure - just added an explicit self.

[CLAassistant]

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