fix wording in exec.task/p3 about `task<T,E>::error_types`

[ericniebler]

[exec.task]/p3 reads:

3. A program is ill-formed if error_types is not a specialization of completion_signatures<ErrorSigs...> or ErrorSigs contains an element which is not of the form set_error_t(E) for some type E.

completion_signatures<ErrorSigs...> is a class type. i don't know what a specialization of a class type is.