[associative.set.syn] Fix cross-reference link for 'multiset'

Fixes #1424.

Author: tkoeppe

source/containers.tex

@@ -5881,7 +5881,7 @@
               set<Key, Compare, Allocator>& y)
       noexcept(noexcept(x.swap(y)));
 
-  // \ref{set}, class template multiset
+  // \ref{multiset}, class template multiset
   template <class Key, class Compare = default_order_t<Key>,
             class Allocator = allocator<Key>>
     class multiset;