docs: Add missing `end`s to examples in OAuth2 module docs #1063

Job	Run time
mix deps.get	34s
mix hex.audit	28s
MIX_ENV=test mix.compile	2m 19s
mix credo --strict	30s
mix doctor --full --raise	43s
mix dialyzer	4m 48s
mix git_ops.check_message	44s
mix format --check-formatted	44s
mix ash_postgres.generate_migrations --check	48s
mix sobelow --config	41s
mix spark.cheat_sheets --check	42s
mix spark.formatter --check	43s
mix test	1m 18s
mix deps.unlock --check-unused	38s
MIX_ENV=dev mix.compile	0s
mix git-ops.release	0s
mix docs	0s
Deploy docs to GitHub pages.	0s
	15m 40s

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