fix block_entities example #1331

Job	Run time
valence-fmt (ubuntu-latest, default, nightly-2023-06-20)	27s
extractor-tests (windows-latest)	4m 15s
extractor-tests (macos-latest)	6m 15s
extractor-tests (ubuntu-latest)	2m 38s
valence-clippy (windows-latest, default, stable)	9m 20s
valence-clippy (macos-latest, default, stable)	5m 21s
valence-clippy (ubuntu-latest, default, stable)	5m 5s
valence-docs (windows-latest, default, stable)	8m 23s
valence-docs (macos-latest, default, stable)	6m 27s
valence-docs (ubuntu-latest, default, stable)	3m 23s
valence-tests (windows-latest, default, stable)	16m 22s
valence-tests (macos-latest, default, stable)	15m 40s
valence-tests (ubuntu-latest, default, stable)	15m 12s
	1h 38m 48s

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