Correct voice_id to voiceId in Vapi Typescript SDK

[govindup63]

Description

• I was trying to use the Vapi TypeScript API to create a new agent.
• The code snippet listed on the web/documentation uses voice_id, but the SDK expects voiceId (camelCase).
• This mismatch causes a TypeScript error:
  Object literal may only specify known properties, but 'voice_id' does not exist in type 'PlayHtVoice'. Did you mean to write 'voiceId'?
• By the way, if you’re looking for an intern who can catch these things (and bring good vibes), I’m your person! Resume here: https://govindup63.github.io/Resume-Achives/

Testing Steps

• Run the app locally using fern docs dev or navigate to preview deployment
• Ensure that the changed pages and code snippets work