[syserr.fmt] Replace \unicode with \ucode to render U+FFFD

Author: eisenwave

source/diagnostics.tex

@@ -1311,7 +1311,7 @@
 then let \tcode{msg} be \tcode{ec.message()} transcoded to UTF-8
 with maximal subparts of ill-formed subsequences
 substituted with \unicode{fffd}{replacement character}
-per the Unicode Standard, Chapter 3.9 \unicode{fffd} Substitution in Conversion.
+per the Unicode Standard, Chapter 3.9 \ucode{fffd} Substitution in Conversion.
 \item
 Otherwise, let \tcode{msg} be \tcode{ec.message()}
 transcoded to an