simplify archive regex

[labkode]

https://github.com/cs3org/reva/pull/2235#pullrequestreview-801039778

[defending1]

In #2235 (review) @ishank011 mentioned that the current regex is /eos/user/[a-z]/[a-z]./\w. Is there a place where the format of accepted folders is described in the docs?

I also see another problem about this expression. A case that this expression would accept is /eos/user/a/a;/3? which is not allowed.