docs: sim: fix typo async for #3547

Job	Run time
smoke (amaranth-lang/amaranth-boards)	34s
document	50s
smoke (amaranth-lang/amaranth-stdio)	36s
check-links	44s
test (3.8)	2m 22s
smoke (amaranth-lang/amaranth-soc)	36s
test (3.9)	2m 15s
test (3.10)	2m 11s
test (3.11)	2m 39s
test (3.12)	3m 28s
test (pypy-3.8)	5m 1s
test (pypy-3.9)	4m 40s
test (pypy-3.10)	4m 47s
test (3.13-dev)	3m 14s
publish-docs	14s
publish-docs-dev	0s
required	1s
publish-schemas	12s
publish-package	45s
publish-release	0s
	35m 9s

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