[task.promise] Fix erroneous "Allocator" that should be "allocator_type"

This was a mistake in the paper (a leftover from a previous revision).

Author: tkoeppe

source/exec.tex

@@ -7648,11 +7648,11 @@
 \begin{itemdescr}
 \pnum
 If there is no parameter with type \tcode{allocator_arg_t} then let
-\tcode{alc} be \tcode{Allocator()}.
+\tcode{alc} be \tcode{allocator_type()}.
 Otherwise, let \tcode{arg_next} be the parameter
 following the first \tcode{allocator_arg_t} parameter,
-and let \tcode{alc} be \tcode{Allocator(arg_next)}.
-Then \tcode{PAlloc} is \tcode{allocator_traits<Allocator>::template
+and let \tcode{alc} be \tcode{allocator_type(arg_next)}.
+Then \tcode{PAlloc} is \tcode{allocator_traits<allocator_type>::template
 rebind_alloc<U>}, where \tcode{U} is an unspecified type
 whose size and alignment are both \mname{STDCPP_DEFAULT_NEW_ALIGNMENT}.