doc: fix small typo

Corrected the phrasing from "Checkout" to "Check out" for clarity. ("to check something out", in this case the code, VS "checkout" as the place where someone pays for their purchases)

Author: florentdrousset

symfony/index.md

@@ -815,7 +815,7 @@ Keep in mind that you can use your favorite client-side technology: API Platform
 requests is OK (even COBOL can do that).
 
 To go further, the API Platform team maintains a demo application showing more advanced use cases like leveraging serialization
-groups, user management, or JWT and OAuth authentication. [Checkout the demo code source on GitHub](https://github.com/api-platform/demo)
+groups, user management, or JWT and OAuth authentication. [Check out the demo code source on GitHub](https://github.com/api-platform/demo)
 and [browse it online](https://demo.api-platform.com).
 
 ## Screencasts