[polymorphic.ctor] Fix wrong \indexlibraryctor

Author: abhinavagarwal07

source/memory.tex

@@ -6796,7 +6796,7 @@
 using the allocator \exposid{alloc}.
 \end{itemdescr}
 
-\indexlibraryctor{indirect}%
+\indexlibraryctor{polymorphic}%
 \begin{itemdecl}
 constexpr explicit polymorphic(allocator_arg_t, const Allocator& a);
 \end{itemdecl}