Docs request: update `README` defaults

[jamesbraza]

The README says gpt-3.5-turbo:

vim-chatgpt/README.md

Line 62 in 343830d

- g:chat_gpt_model: This option allows you to specify the ChatGPT model you'd like to use. By default, it is set to 'gpt-3.5-turbo' with a token limit of 4097, If you prefer to use a different model, such as {"gpt-3.5-turbo-16k": 16385, "gpt-4": 8192, "gpt-4-32k": 32768}, simply change the value to the desired model name. Note that using a different model may affect the quality of the results and API usage costs.

The code says gpt-4o:

vim-chatgpt/plugin/chatgpt.vim

Line 19 in 343830d

let g:chat_gpt_model = 'gpt-4o'

The README needs to have its values updated

[CoderCookE]

i've updated the readme