fix observer docs (#15415) #15

Job	Run time
build (windows-latest)	9m 29s
check-bevy-internal-imports	6s
build (ubuntu-latest)	4m 25s
check-doc	11m 15s
check-missing-examples-in-docs	34s
build (macos-latest)	3m 47s
ci	4m 28s
miri	14m 53s
run-examples-macos-metal	5m 13s
toml	2m 7s
typos	8s
check-missing-features-in-docs	38s
markdownlint	1m 11s
check-compiles	6m 7s
build-wasm	1m 45s
msrv	1m 53s
build-wasm-atomics	1m 56s
	1h 9m 55s

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