fix block_entities example #1332

Job	Run time
valence-fmt (ubuntu-latest, default, nightly-2023-06-20)	48s
extractor-tests (windows-latest)	3m 42s
extractor-tests (macos-latest)	3m 18s
extractor-tests (ubuntu-latest)	3m 9s
valence-clippy (windows-latest, default, stable)	8m 48s
valence-clippy (macos-latest, default, stable)	9m 8s
valence-clippy (ubuntu-latest, default, stable)	5m 11s
valence-docs (windows-latest, default, stable)	10m 42s
valence-docs (macos-latest, default, stable)	5m 31s
valence-docs (ubuntu-latest, default, stable)	4m 2s
valence-tests (windows-latest, default, stable)	23m 24s
valence-tests (macos-latest, default, stable)	19m 39s
valence-tests (ubuntu-latest, default, stable)	15m 0s
	1h 52m 22s

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