lib: fix clippy and doc — radicle-tui

Applied two fixes – removing the unnecessary use of to_string and using direct URL <> syntax.

 }
 /// FIXME Remove workaround after a new `ratatui` version with
 /// https://github.com/ratatui-org/ratatui/pull/981/ included was released.
 /// <https://github.com/ratatui-org/ratatui/pull/981/> included was released.
 pub struct TermionBackendExt<W>
 where
     W: Write,

         let mut actual = String::new();
         for (idx, line) in lines.iter().enumerate() {
             if idx == lines.len() - 1 {
                 actual.push_str(&line.to_string());
                 actual.push_str(line);
             } else {
                 actual.push_str(&format!("{}\n", line));
             }