fix: Readme Typo

[milldr]

What

corrected complete() to complete_prompt()

Why

Incorrect method name:

error[E0599]: no method named `complete` found for struct `Client` in the current scope
  --> src/main.rs:11:16
   |
11 |         client.complete(prompt.as_str()).await?
   |                ^^^^^^^^ help: there is a method with a similar name: `complete_prompt`

[deontologician]

Thanks!