Wrong minimum iOS version listed in MDC section

[SupDos]

https://github.com/altstoreio/FAQ/blob/main/how-to-use-altstore/macdirtycow.md

The minimum iOS version needed to remove the 3-app limit is iOS 15 as the app does not support iOS 14

[ssdrive-flaktra]

made a pull request to fix this, #5