[class.prop] point to actual layout spec in [expr.rel]

The original ref does not define the layout (anymore?), thanks to Jens for providing me with the correct ref!

Author: Axel-Naumann

source/classes.tex

@@ -257,7 +257,7 @@
 \begin{note}
 Standard-layout classes are useful for communicating with
 code written in other programming languages. Their layout is specified
-in~\ref{class.mem}.
+in~\ref{expr.rel}.
 \end{note}
 
 \pnum