Fix apk installation command in README #410

Job	Run time
docker-master	1h 41m 15s
publish-standalone-dry-run	15s
test-standalone (master, dev, default)	1m 3s
build-wasm-of-ocaml (master, 4.14.1)	13m 28s
build-js-of-ocaml (master, 4.11.1)	14m 23s
generated-files (master)	37s
validate-docker-master	34m 47s
test-amd64	2h 18m 54s
standalone-haskell (master)	53m 12s
deploy-js-wasm-of-ocaml	43s
docker-check-all	6s
	5h 58m 43s

Provide feedback