Remove unnecessary ResMut in examples #33297

Job	Run time
ci	1m 57s
miri	6m 51s
toml	4s
build (windows-latest)	3m 8s
check-bevy-internal-imports	4s
build (ubuntu-latest)	3m 13s
check-doc	5m 20s
build (macos-latest)	4m 2s
check-missing-examples-in-docs	31s
run-examples-on-windows-dx12	11m 33s
check-missing-features-in-docs	34s
markdownlint	1m 20s
check-compiles	2m 20s
msrv	46s
build-wasm	38s
	42m 21s

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