docs: clarify comment in owned_mutation test (fix #2303)

[KaustubhOG]

Closes #2303

This PR updates the outdated comment in the owned_mutation test
within exercises/19_smart_pointers/cow1.rs to better explain how
to_mut() behaves when the data is already owned.

[KaustubhOG]

Hi!
This PR addresses #2303 by clarifying the comment in the owned_mutation test.
All checks have passed and there are no merge conflicts.
Please review when convenient — happy to make any adjustments if needed.
Thanks!

[KaustubhOG]

Hi!

All checks have passed, there are no merge conflicts, and the PR fully addresses the linked issue.
Whenever you have a moment, @rust-lang/rustlings-maintainers, could you please review this?

Happy to make any updates or adjustments if needed. Thanks for your time and work on Rustlings! 🙌

[mo8it]

I don't think that the new description is much better. Please stop annoying maintainers by pinging them repeatedly.

[KaustubhOG]

Thanks for the feedback — and I’m truly sorry for the repeated pings.
I didn’t mean to add pressure, and I appreciate the clarification.
I’m still new to open-source and learning the etiquette.

It won’t happen again. Thanks for your time.