[meta.reflection.define.aggregate] Fix typo ("C" should be "D").

This seems to have been a mistake during application of P2996R13.

Author: tkoeppe

source/meta.tex

@@ -6502,7 +6502,7 @@
 
 \pnum
 \effects
-Produces an injected declaration $C$\iref{expr.const}
+Produces an injected declaration $D$\iref{expr.const}
 that defines $C$ and has properties as follows:
 \begin{itemize}
 \item