lab5: Mention use of std::io::BufRead in the Rust template

hugo/content/docs/cviceni/lab5/_index.md

@@ -120,6 +120,8 @@ potřeba uvolnit funkcí `free()`.
 {{< /tab >}}
 {{< tab "Rust" >}}
 ```rust
+use std::io::BufRead;
+
 for line in std::io::stdin().lock().lines().map(|l| l.unwrap()) {
     if let Some((num, word)) = line.split_once(' ') {
         let x = num.parse::<usize>()?;