[diff.cpp23.strings] fix copy&paste error

Co-authored-by: Johel Ernesto Guerrero Peña <[email protected]>

Author: jwakely

source/compatibility.tex

@@ -11,7 +11,7 @@
 ISO \CppXXIII{} (ISO/IEC 14882:2023, \doccite{Programming Languages --- \Cpp{}}),
 by the chapters of this document.
 
-\rSec2[diff.cpp23.lex]{\ref{strings}: strings library}
+\rSec2[diff.cpp23.strings]{\ref{strings}: strings library}
 
 \diffref{string.conversions}
 \change