Section 6.1 example: no external crate `utf8_slice`

[Sergei Tikhomirov]

Running utf8_slice example in Section 6.1 produces an error:

   Compiling playground v0.0.1 (/playground)
error[E0432]: unresolved import `utf8_slice`
 --> src/main.rs:1:5
  |
1 | use utf8_slice;
  |     ^^^^^^^^^^ no external crate `utf8_slice`

For more information about this error, try `rustc --explain E0432`.
error: could not compile `playground` due to previous error