[tab:lex.charset.literal] Shorten table heading

Fixes ISO/CS comment (C++23 proof)

Author: jensmaurer

source/lex.tex

@@ -410,7 +410,7 @@
 all characters of the basic character set,
 plus the control characters specified in \tref{lex.charset.literal}.
 
-\begin{floattable}{Additional control characters in the basic literal character set}{lex.charset.literal}{ll}
+\begin{floattable}{Additional control characters}{lex.charset.literal}{ll}
 \topline
 \ohdrx{2}{character} \\ \capsep
 \ucode{0000} & \uname{null} \\