[flat.multiset.modifiers] Add missing constexpr (#8675)

Author: hewillk

source/containers.tex

@@ -20174,7 +20174,7 @@
 \indexlibrarymember{insert_range}{flat_multiset}%
 \begin{itemdecl}
 template<@\exposconcept{container-compatible-range}@<value_type> R>
-  void insert_range(R&& rg);
+  constexpr void insert_range(R&& rg);
 \end{itemdecl}
 
 \begin{itemdescr}