docs: fix options validator docs example #944

Job	Run time
ash-ci / mix deps.get	23s
ash-ci / mix igniter.upgrade	4s
ash-ci / audit	37s
ash-ci / MIX_ENV=test mix.compile	42s
ash-ci / mix spark.formatter --check	4s
ash-ci / mix format --check-formatted	29s
ash-ci / mix credo --strict	4s
ash-ci / mix docs	1m 9s
ash-ci / mix sobelow	26s
ash-ci / mix spark.cheat_sheets --check	24s
ash-ci / mix deps.unlock --check-unused	36s
ash-ci / MIX_ENV=dev mix.compile	25s
ash-ci / mix ash.codegen --check	0s
ash-ci / mix test	56s
ash-ci / mix dialyzer	2m 55s
ash-ci / Deploy docs to GitHub pages.	7s
ash-ci / NIF ${{ matrix.nif }} - ${{ matrix.job.target }} (${{ matrix.job.os }})	0s
ash-ci / Release	0s
	9m 21s

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