Updated `AppFlags.swift` to `AppFlags.self` in Vexillographer.md

[will-lumley]

📒 Description

I noticed that in the DocC that instead of AppFlags.self there was AppFlags.swift. This is just a tiny PR to correct that.

🔍 Detailed Design

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📓 Documentation Plan

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🗳 Test Plan

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🧯 Source Impact

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~### ✅ Checklist~

[sonarqubecloud[bot]]

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