chore: add FFFD to spell-checker ignore list in fmt test.

Author: mattsu2020

tests/by-util/test_fmt.rs

@@ -3,7 +3,7 @@
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
 
-// spell-checker:ignore plass samp
+// spell-checker:ignore plass samp FFFD
 #[cfg(target_os = "linux")]
 use std::os::unix::ffi::OsStringExt;
 use uutests::new_ucmd;