FR-033-335 33.9.12 [exec.adapt] Drop std:: from headlines

[wg21bot]

The headings for [exec.associate], [exec.stop.when], [exec.spawn.future], [exec.spawn] use std::execution::foo, the rest of the clause only execution::foo.

Proposed change:

Remove the redundant std:: in the specified headings.

[jensmaurer]

Accepted.