[version.syn] Replace 'C++' with 'this document'

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

Author: jensmaurer

source/support.tex

@@ -546,7 +546,7 @@
 after inclusion of any member of the set of library headers
 indicated in the corresponding comment in this synopsis.
 \begin{note}
-Future revisions of \Cpp{} might replace
+Future revisions of this document might replace
 the values of these macros with greater values.
 \end{note}