docs: Fix example of `as_struct` #11812

Job	Run time
check-features	8m 40s
check-wasm	6m 19s
integration-test (ubuntu-latest)	1m 55s
integration-test (windows-latest)	5m 38s
test (ubuntu-latest)	3m 46s
test (windows-latest)	10m 28s
	36m 46s

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