[task.promise] Refer to parameter types of the completion signatures

Fixes NB US 260-390 (C++26 CD).

Author: jensmaurer

source/exec.tex

@@ -7478,7 +7478,7 @@
 \pnum
 \exposid{error-variant} is a \tcode{variant<monostate,
 remove_cvref_t<E>...>}, with duplicate types removed, where \tcode{E...}
-are template arguments of the specialization of
+are the parameter types of the template arguments of the specialization of
 \tcode{execution::completion_signatures} denoted by
 \tcode{error_types}.