Fix model provider typo in workflows.mdx (#525)

Author: Paul-Cornell

api-reference/workflow/workflows.mdx

@@ -598,7 +598,7 @@ Allowed values for `provider` and `model` include:
 
   - `"model": "claude-3-5-sonnet-20241022"`
 
-- `"provider": "anthropic""openai"`
+- `"provider": "openai"`
 
   - `"model": "gpt-4o"`