Fix doc sample for Cursor

Fixes #29219

Author: wesleywiser

src/libstd/io/cursor.rs

@@ -69,7 +69,7 @@ use slice;
 ///     use std::io::Cursor;
 ///     let mut buff = Cursor::new(vec![0; 15]);
 ///
-///     write_ten_bytes(&mut buff).unwrap();
+///     write_ten_bytes_at_end(&mut buff).unwrap();
 ///
 ///     assert_eq!(&buff.get_ref()[5..15], &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
 /// }