[dcl.fct.def.coroutine] p11 Missing "noexcept" on final_suspend in example

[jensmaurer]

[dcl.fct.def.coroutine] p15 says "The expression co_await promise .final_suspend() shall not be potentially-throwing (14.5).", so the example needs to say "noexcept" for the final_suspend declaration.

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