[associative.map.syn] Fix indentation (#8673)

Author: Andreas-Krug

source/containers.tex

@@ -11525,7 +11525,7 @@
 
   template<class Key, class T, class Compare, class Allocator>
     constexpr void swap(multimap<Key, T, Compare, Allocator>& x,
-              multimap<Key, T, Compare, Allocator>& y)
+                        multimap<Key, T, Compare, Allocator>& y)
       noexcept(noexcept(x.swap(y)));
 
   // \ref{multimap.erasure}, erasure for \tcode{multimap}