[atomics.types.pointer] Fix parameter type (should be "see above")

A misapplication of P3111R8.

Author: tkoeppe

source/threads.tex

@@ -5930,8 +5930,8 @@
 \indexlibrarymember{store_min}{atomic<T*>}%
 \indexlibrarymember{store_sub}{atomic<T*>}%
 \begin{itemdecl}
-void store_@\placeholdernc{key}@(ptrdiff_t operand, memory_order order = memory_order::seq_cst) volatile noexcept;
-constexpr void store_@\placeholdernc{key}@(ptrdiff_t operand, memory_order order = memory_order::seq_cst) noexcept;
+void store_@\placeholdernc{key}@(@\seeabovenc@ operand, memory_order order = memory_order::seq_cst) volatile noexcept;
+constexpr void store_@\placeholdernc{key}@(@\seeabovenc@ operand, memory_order order = memory_order::seq_cst) noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}