Update quickstart.mdx (#45115)

## I have read the
[CONTRIBUTING.md](https://github.com/supabase/supabase/blob/master/CONTRIBUTING.md)
file.

YES

## What kind of change does this PR introduce?

docs update


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Added a curl command example in the API quickstart guide,
demonstrating how to query the todos endpoint with proper authentication
headers for quick API testing.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

apps/docs/content/guides/api/quickstart.mdx

@@ -200,6 +200,14 @@ You can query the route in your browser, by appending the `publishable` key as a
 
 `https://<PROJECT_REF>.supabase.co/rest/v1/todos?apikey=<PUBLISHABLE_KEY>`
 
+### Curl
+
+```
+curl 'https://<PROJECT_REF>.supabase.co/rest/v1/todos?select=*' \
+  -H "apikey: <PUBLISHABLE_KEY>" \
+  -H "Authorization: Bearer <PUBLISHABLE_KEY>"
+```
+
 ### Client libraries
 
 We provide a number of [Client Libraries](https://github.com/supabase/supabase#client-libraries).