[meta.unary.prop] Fix index entry for is_aggregate. (#1581)

Author: Eelis

source/utilities.tex

@@ -15926,7 +15926,7 @@
  can be marked with \tcode{final}. \end{note}                                        &
  If \tcode{T} is a class type, \tcode{T} shall be a complete type.                          \\ \rowsep
 
-\indexlibrary{\idxcode{is_aggregate_signed}!class}%
+\indexlibrary{\idxcode{is_aggregate}}%
 \tcode{template <class T>}\br
   \tcode{struct is_aggregate;}           &
  \tcode{T} is an aggregate type~(\ref{dcl.init.aggr}) &