garbage-collection guide: noop read_record

[reynir]

The following read_record function doesn't do anything useful:

ocaml.org/data/tutorials/guides/1wf_05_garbage_collection.md

Lines 178 to 181 in f4b7302

	let read_record record n fd =
	seek_record n fd;
	ignore (Unix.read fd (Bytes.of_string record.name) 0 name_size);
	ignore (Unix.read fd (Bytes.of_string record.address) 0 addr_size)

The name and addr fields are copied into bytes, and the data is read into those bytes. However, the bytes are just thrown away and does nothing useful. Furthermore, nothing is done to ensure all is read.