[lex.charset] Clarify normative reference to Unicode for UTF-x

Fixes ISO/CS 008 (C++23 DIS).

Author: jensmaurer

source/lex.tex

@@ -449,9 +449,10 @@
 \indextext{UTF-16}%
 \indextext{UTF-32}%
 For a UTF-8, UTF-16, or UTF-32 literal,
+the implementation shall encode
 the Unicode scalar value
 corresponding to each character of the translation character set
-is encoded as specified in the Unicode Standard
+as specified in the Unicode Standard
 for the respective Unicode encoding form.
 \indextext{character set|)}