Fix wrong llm value in example (#1929)

Original example had `mixtal-llm` which would result in an error.
Replaced with gpt-4o according to https://docs.crewai.com/concepts/llms

docs/quickstart.mdx

@@ -278,7 +278,7 @@ email_summarizer:
       Summarize emails into a concise and clear summary
     backstory: >
       You will create a 5 bullet point summary of the report
-    llm: mixtal_llm
+    llm: openai/gpt-4o
 ```
 
 <Tip>