✨ Update error message for missing OpenAI API key 🔑

The error message for missing OpenAI API key is updated, now it indicates the
address where the developers can obtain their API key.
It is important that developers can know how to obtain the API key to use the
API service to avoid accumulating errors, and frustrate developers.

main.go

@@ -68,7 +68,7 @@ func getStagedDiff() (string, error) {
 
 func generateText(prompt string) (string, error) {
 	if apiKey == "" {
-		return "", fmt.Errorf("OPENAI_API_KEY environment variable is not set")
+		return "", fmt.Errorf("OPENAI_API_KEY environment variable is not set. you can get it from https://platform.openai.com/account/api-keys.")
 	}
 	url := "https://api.openai.com/v1/chat/completions"
 	messages := []Message{