[DOC]: Update LLM example to default to OpenAI

[exactlyallan]

How would you describe the priority of this documentation request

Medium

Please provide a link or source to the relevant docs

https://github.com/nv-morpheus/Morpheus/tree/branch-24.06/examples/llm/completion#options

Describe the problems in the documentation

Current LLM example defaults to NemoLLM. Default should switch to OpenAI for better accessibility. Also would be helpful to link to openAI quickstart to help guide to key setup.

`--llm_service [NemoLLM|OpenAI]

Description: LLM service to issue requests to.
Default: NemoLLM`

(Optional) Propose a correction

No response

Code of Conduct

• I agree to follow this project's Code of Conduct
• I have searched the open documentation issues and have found no duplicates for this bug report