docs: clarify stale comment in cow1 owned_mutation test

[DeoJin]

Summary

• update the stale comment in exercises/19_smart_pointers/cow1.rs
• clarify that o_mut() returns a mutable reference to already-owned data here
• remove the misleading implication that �bs_all itself returns a reference

Closes #2303

Validation

• unable to run cargo test owned_mutation --bin rustlings in this environment because Rust tooling (cargo,
  ustc) is not installed
• verified the change is comment-only and scoped to the referenced exercise text