Fix typo in fast branch explanation

Author: exPHAT

README.md

@@ -117,7 +117,7 @@ func convertAudioFileToPCMArray(fileURL: URL, completionHandler: @escaping (Resu
 
 You may find the performance of the transcription slow when compiling your app for the `Debug` build configuration. This is because the compiler doesn't fully optimize SwiftWhisper unless the build configuration is set to `Release`.
 
-You can get around this by installing a version of SwiftWhisper that uses `.unsafeFlags(["-03"])` to force a maximum optimization. The easiest way to do this is to use the latest commit on the [`fast`](https://github.com/exPHAT/SwiftWhisper/tree/fast) branch. Alternatively, you can configure your scheme to build in the `Release` configuration.
+You can get around this by installing a version of SwiftWhisper that uses `.unsafeFlags(["-O3"])` to force maximum optimization. The easiest way to do this is to use the latest commit on the [`fast`](https://github.com/exPHAT/SwiftWhisper/tree/fast) branch. Alternatively, you can configure your scheme to build in the `Release` configuration.
 
 ```swift
   ...