[specialized.algorithms] Rename voidify's parameter

`ptr` is an odd name for a parameter that is a reference to storage for an object.

Author: CaseyCarter

source/utilities.tex

@@ -8105,8 +8105,8 @@
 \tcode{\placeholdernc{voidify}}:
 \begin{codeblock}
 template<class T>
-  void* @\placeholdernc{voidify}@(T& ptr) noexcept {
-    return const_cast<void*>(static_cast<const volatile void*>(addressof(ptr)));
+  void* @\placeholdernc{voidify}@(T& obj) noexcept {
+    return const_cast<void*>(static_cast<const volatile void*>(addressof(obj)));
   }
 \end{codeblock}