Update spell-checker ignore list in date.rs

Author: sylvestre

src/uu/date/src/date.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 strtime ; (format) DATEFILE MMDDhhmm ; (vars) datetime datetimes getres AWST ACST AEST foobarbaz
+// spell-checker:ignore strtime ; (format) DATEFILE MMDDhhmm ; (vars) datetime datetimes getres AWST ACST AEST foobarbaz FFFD
 
 mod format_modifiers;
 mod locale;