Suggestion: Mandate trailing slashes for directories

[multimeric]

We already mandate that the root data entity has a trailing slash:

ro-crate/docs/_specification/1.1/root-data-entity.md

Line 117 in cee19f1

* `@id`: MUST end with `/` and SHOULD be the string `./`

Also, there are examples that use subdirectories also with trailing slashes:

ro-crate/docs/_specification/1.1/data-entities.md

Lines 105 to 110 in cee19f1

	{
	"@id": "lots_of_little_files/",
	"@type": "Dataset",
	"name": "Too many files",
	"description": "This directory contains many small files, that we're not going to describe in detail."
	}

However, it is not mandated in the spec that directories have a trailing slash. I think this should be standardised.