Fix `animation_masks` example's buttons #51327

Job	Run time
miri	9m 49s
build (windows-latest)	5m 58s
ci	2m 41s
run-examples-macos-metal	6m 24s
build (ubuntu-latest)	2m 44s
toml	2m 1s
build (macos-latest)	2m 4s
typos	8s
check-doc	9m 10s
check-bevy-internal-imports	5s
check-missing-examples-in-docs	29s
check-missing-features-in-docs	32s
markdownlint	1m 5s
check-compiles	4m 49s
check-compiles-no-std	33s
build-wasm	59s
build-wasm-atomics	1m 33s
msrv	59s
	52m 3s

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