[meta.reflection.define.aggregate] Fix note text and !=/==.

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

Author: tkoeppe

source/meta.tex

@@ -6493,12 +6493,9 @@
   \item
     \tcode{$N_K$ != $N_L$} is \tcode{true} or
   \item
-    \tcode{$N_K$ != u8"_"} is \tcode{true}.
+    \tcode{$N_K$ == u8"_"} is \tcode{true}.
     \begin{note}
-    $C$ can be a class template specialization
-    for which there is a reachable definition of the class template.
-    In this case,
-    the injected declaration is an explicit specialization.
+    Every provided identifier is unique or \tcode{"_"}.
     \end{note}
   \end{itemize}
 \end{itemize}