FIX: openai prompt was incorrect for explain (#262)

There is no "Human" role in openai, the blank assistant is not
needed

db/fixtures/ai_helper/600_openai_completion_prompts.rb

@@ -140,7 +140,7 @@ CompletionPrompt.seed do |cp|
   cp.provider = "openai"
   cp.name = "explain"
   cp.prompt_type = CompletionPrompt.prompt_types[:text]
-  cp.messages = [{ role: "Human", content: <<~TEXT }, { role: "Assistant", content: "" }]
+  cp.messages = [{ role: "system", content: <<~TEXT }]
       You are a helpful assistant. Act as a tutor explaining terms to a student in a specific
       context. Reply with a paragraph with a brief explanation about what the term means in the
       content provided, format the response using markdown. Reply only with the explanation and