failed to load external entity "relaxng/"

I'm in the peardoc directory and have the xmllint executable there, as well.

I then type in the following:

xmllint --valid --noout --noent --relaxng relaxng/ en/bookinfo.xml

And I get the following:

error: failed to load external entity "relaxng/"
xmlRelaxNGParse: could not load relaxng/
Relax-NG schema relaxng/ failed to compile
...and then a bunch of 'Entity not defined' errors.

The relaxng folder contains all the files found in this *.zip:

http://www.docbook.org/rng/4.3/docbook-rng-4.3.zip

Any ideas as to what I'm doing wrong?