[lex.charset] Add reference to the Unicode Standard

Author: jensmaurer

source/lex.tex

@@ -247,7 +247,8 @@
 The \defnadj{translation}{character set} consists of the following elements:
 \begin{itemize}
 \item
-each abstract character assigned a code point in the Unicode codespace, and
+each abstract character assigned a code point in the Unicode codespace
+as specified in the Unicode Standard, and
 \item
 a distinct character for each Unicode scalar value
 not assigned to an abstract character.