fix zsh completion guidance

We upgraded to clap 4.1 in 8f1dc49 (cargo: upgrade to clap 4.1,
2023-03-17), and the pipe to `sed` was removed in the README.md file in
that same commit. However the documentation was not updated in mod.rs,
which leads us to give the obsolete (now incorrect) advice when we run
`jj util completion --help`.

See #1393.

src/commands/mod.rs

@@ -993,7 +993,7 @@ struct UtilCompletionArgs {
     ///
     /// autoload -U compinit
     /// compinit
-    /// source <(jj util completion --zsh | sed '$d')  # remove the last line
+    /// source <(jj util completion --zsh)
     /// compdef _jj jj
     #[arg(long, verbatim_doc_comment)]
     zsh: bool,