chore(provider): use latest cohere models (#400)

Update prompts to correct grammar
concat with table.concat to avoid overhead

Signed-off-by: Aaron Pham <contact@aarnphm.xyz>

lua/avante/providers/openai.lua

@@ -28,10 +28,7 @@ local M = {}
 M.api_key_name = "OPENAI_API_KEY"
 
 M.parse_message = function(opts)
-  local user_prompt = ""
-  for _, user_prompt_ in ipairs(opts.user_prompts) do
-    user_prompt = user_prompt .. "\n\n" .. user_prompt_
-  end
+  local user_prompt = table.concat(opts.user_prompts, "\n\n")
 
   ---@type string | OpenAIMessage[]
   local user_content