BG180 17.12.05 Fix example using await_suspend

[wg21bot]

(BG-local 4)

(Related to BG2) The code example uses the void-returning variant of await_suspend().

Proposed change:
Change suspend_never::await_suspend() to return its argument and change suspend_always::await_suspend() to return nullptr.

[mihaylovm]

This issue is related to BG049 #48.

[jfbastien]

Since this is related to BG049, moving to EWG.

[jfbastien]

Given the vote on #48, this cannot be changed. EWG agreed unanimously to do no change.

[jensmaurer]

Rejected. There was no consensus to adopt this change.