docs: sim: fix typo async for #3546

Job	Run time
document	51s
check-links	44s
smoke (amaranth-lang/amaranth-boards)	34s
smoke (amaranth-lang/amaranth-stdio)	38s
smoke (amaranth-lang/amaranth-soc)	45s
test (3.8)	2m 23s
test (3.9)	2m 21s
test (3.10)	2m 6s
test (3.11)	2m 45s
test (3.12)	3m 24s
test (pypy-3.8)	5m 2s
test (pypy-3.9)	4m 47s
test (pypy-3.10)	4m 40s
test (3.13-dev)	3m 3s
publish-docs	5s
publish-docs-dev	0s
required	1s
publish-package	37s
publish-schemas	6s
publish-release	0s
	34m 52s

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