Fix comment to mention the correct type of error

[gurjeet]

Out-of-bounds indexing leads to compile-time error, as shown in below output from the playground run of this code snippet.

   Compiling playground v0.0.1 (/playground)
error: this operation will panic at runtime
  --> src/main.rs:54:20
   |
54 |     println!("{}", xs[5]);
   |                    ^^^^^ index out of bounds: the length is 5 but the index is 5
   |
   = note: `#[deny(unconditional_panic)]` on by default

error: could not compile `playground` due to previous error

[rustbot]

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[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @marioidival (or someone else) soon.