fix: add missing closing `]` in guide #1016

Job	Run time
ash-ci / audit	34s
ash-ci / mix deps.get	23s
ash-ci / mix igniter.upgrade	30s
ash-ci / MIX_ENV=test mix.compile	28s
ash-ci / mix docs	30s
ash-ci / MIX_ENV=dev mix.compile	21s
ash-ci / mix credo --strict	4s
ash-ci / mix ash.codegen --check	0s
ash-ci / mix dialyzer	3m 49s
ash-ci / mix format --check-formatted	23s
ash-ci / Deploy docs to GitHub pages.	0s
ash-ci / mix sobelow	21s
ash-ci / mix spark.cheat_sheets --check	23s
ash-ci / mix test	46s
ash-ci / mix spark.formatter --check	7s
ash-ci / mix deps.unlock --check-unused	31s
ash-ci / Release	0s
	9m 10s

Provide feedback