[task.class] Improve error_types wording

Fixes NB US 247-377 (C++26 CD).

Author: timsong-cpp

source/exec.tex

@@ -7237,8 +7237,9 @@
 
 \pnum
  A program is ill-formed if \tcode{error_types} is not a
-specialization of \tcode{completion_signatures<ErrorSigs...>} or
-\tcode{ErrorSigs} contains an element which is not of the form
+specialization of \tcode{execution::completion_signatures}
+or if the template arguments of that specialization
+contain an element which is not of the form
 \tcode{set_error_t(E)} for some type \tcode{E}.
 
 \pnum