TranscriptionVerbose.duration is a number, not a string

The real API returns numbers, not strings.

[The API is also documented here](https://platform.openai.com/docs/api-reference/audio/verbose-json-object). That documentation partially makes the same mistake: it claims that `duration` is a string. However, it also shows this example: `"duration": 8.470000267028809`, in which the duration is a number.

src/resources/audio/transcriptions.ts

@@ -104,7 +104,7 @@ export interface TranscriptionVerbose {
   /**
    * The duration of the input audio.
    */
-  duration: string;
+  duration: number;
 
   /**
    * The language of the input audio.